<\/p>\nhi, this is chris, the sat expert at magoosh.<\/p>\n
i’ve had over 15 years’ experience helping students ace the sat, and today, i’m going to talk about the distance rate formula. \nwhy do you have to know this?<\/p>\n
well, there’s a certain question type, a word problem about moving vehicles going in different directions, that freaks a lot of people out.<\/p>\n
but this formula can make things a lot easier.<\/p>\n
so here’s a distance problem, we have stephen cycling at a constant rate of 10 miles per hour, and gertrude cycling at a constant rate of 15 miles per hour.<\/p>\n
so we know people are moving at a certain rate for a certain amount of time, and that’s how we know we’re in the distance formula world.<\/p>\n
so we’re gonna come back to this question in a second, and we’re gonna go straight to that distance formula.<\/p>\n
we have distance = rate x time.<\/p>\n
now, we’re gonna put this to work with us, with that very problem we just saw, but i’m gonna show you an easy way to remember this.<\/p>\n
see how d stands for distance, rate stands for r at the beginning, and time starts with t, so there it is?<\/p>\n
and we’re gonna make that, and simplify that into d = rt, that’s gonna be your magic formula.<\/p>\n
you always wanna keep that in mind, but of course, know what those things stand for.<\/p>\n
okay, so let’s take a look back here at stephen and gertrude.<\/p>\n
and not only am i going to give you that question, but i’m gonna give you some answer choices.<\/p>\n
you can try this on your own if you want, but let’s do it together now.<\/p>\n
so stephen cycles at a constant rate of 10 miles an hour for two hours, and then stops, so let’s use our formula.<\/p>\n
w know the rate, that’s the speed, so that’s 10 miles an hour for stephen, let’s get that right there.<\/p>\n
and then he goes for how long, for two hours.<\/p>\n
so we know that if he’s going at this rate ,10 times 2, we have a distance of 20, and that’s essentially how far stephen has gone.<\/p>\n
now, if we look at gertrude, we can do the same math, but she’s going faster, and the question is, how long will it take her?<\/p>\n
so we actually have the distance she needs to go, which is 20, and her speed, which is 15.<\/p>\n
and now, we suddenly don’t know t, that’s what the question is asking for.<\/p>\n
but we can plug this information back into that original equation, and that’s why it’s so handy.<\/p>\n
essentially, we use it twice, once for stephen, we figured out how far he had gone.<\/p>\n
and then we plugged that 20 down into the equation here, with gertrude, and that gave us 20.<\/p>\n
and then that equals 15, being the rate, t is the time.<\/p>\n
we solve for t, next step, here, t is 20 over 15.<\/p>\n
that is equal to 4 over 3, so what is 4 over 3, it’s the same as 1 and one-third, make that 4 a little bit better there.<\/p>\n
1 and one-third, what’s one-third of an hour, 20 minutes, and therefore, 1 hour and 20 minutes it is.<\/p>\n
now, you don’t even have to figure out that one-third being 20 minutes.<\/p>\n
if you look at the answer choices, you have 45 minutes ,which is less than an hour.<\/p>\n
and you look at your, the number we got, four-thirds, that’s greater than 1 but less than 2.<\/p>\n
and the only answer that would work is only answer choice c, so quick way of doing things, as well.<\/p>\n
but the focus, of course, here is the distance-rate formula, let’s do some more.<\/p>\n
this one’s gonna be a little bit harder, scarier, it’s the two trains, the dreaded scenario.<\/p>\n
they are setting off from different cities located 300 miles from each other.<\/p>\n
and they’re headed directed towards each other, but don’t worry, on different tracks.<\/p>\n
the first train moves at a constant rate of 40 miles an hour, and the second moves at a constant rate of 60 miles an hour.<\/p>\n
so what’s the key here, if objects are heading towards each other, we always combine their rate.<\/p>\n
again, if they’re headed directly at each other, you combine their speeds, and so 40 plus 60, that’s gonna be our rate, that’s 100.<\/p>\n
and so we have the fact that they are d, 300 miles from each other.<\/p>\n
and that they are going at 100 miles an hour, cuz we have to combine the rates, and we have to figure out the time.<\/p>\n
and we get 300 = 100t, and then therefore, we can get t = 3.<\/p>\n
and so therefore, we can see that in 3 hours, they’ve gone how far?<\/p>\n
well, in three hours, again, you’re combining their speeds together.<\/p>\n
so this train over here goes 120 miles, which is 40 times 3, this train over here goes 180 miles, so all in all, they’ve traveled 300 miles.<\/p>\n
now, it would seem like the answer is c, 1 o’clock.<\/p>\n
but notice that we’re not waiting for the two trains to get to the other cities, we’re asking when they meet each other.<\/p>\n
and so they’re going to meet each other at this halfway point in time.<\/p>\n
so in three hours, they’ve covered the total 300 miles, but half of 3 hours, which is 1.5 hours, they’ve actually connected.<\/p>\n
that’s the point they intersect, so it’s actually half of 3, which gives us an hour and a half.<\/p>\n
if they started at 10, that means that 11:30, they are going to pass each other, and that’s what makes it tricky.<\/p>\n
now, this is definitely a harder question, but the point is that we can still use distance is equal to rate times time.<\/p>\n
we can still use that formula when we know that we’re combining rates, things coming together.<\/p>\n
but now we’re gonna look at a different problem, the last problem here, which is a tricky one.<\/p>\n
here, we’re going to go back to stephen and gertrude, but we’re gonna put them in cars, instead of bicycles, but here, one is chasing the other one.<\/p>\n
and so unlike the trains, when you’re combining, because they’re heading towards each other, here they’re in catch-up mode, or at least gertrude is.<\/p>\n
so stephen, let’s read the problem here, leaves farm town, driving at a rate of 40 miles an hour, so where is he in one hour?<\/p>\n
well, he’s 40 hours from gertrude, who starts driving at that point at 50 miles an hour, so when is she actually gonna reach stephen?<\/p>\n
well, assuming again that they’re both continuing to drive, after one hour, he is 40 miles an hour, we know that.<\/p>\n
but she is, what, 50 miles an hour, which is 10 miles an hour faster, let me get that 0 there, 10 miles an hour faster than stephen.<\/p>\n
so you can think of it this way, for every hour, she catches up.<\/p>\n
so in one hour, she catches up 10 miles to him, two hours, she catches up 20 miles to him.<\/p>\n
we know that she’s 40 miles behind, so how long will it take her to catch up to him?<\/p>\n
well, she’s gaining 10 miles an hour, that means, and now we can kinda use our distance-rate formula here.<\/p>\n
she’s 40 miles away, and she’s catching up at a rate of 10 miles an hour.<\/p>\n
we can just say, oh, okay, therefore t is equal to 4 hours, that’s the total number of time it will take her to catch.<\/p>\n
so we can still use distance and rate, but we have to think of it differently, in terms of subtracting.<\/p>\n
we’re subtracting, not adding together, which is the tricky part, but there’s our answer.<\/p>\n
and there you have three different setups where we have distance, rate, and time, and we have need to find an answer, relying on our nifty little formula.<\/p>\n
so there we are, you don’t have to be scared anymore of these question types, because you’ve got the distance-rate formula in your back pocket.<\/p>\n
if you like this video, then click on the link in the description below.<\/p>\n
that will take you to sat.www.catharsisit.com, where you can join thousands of other students who are prepping for the sat.<\/p>\n
if you want more helpful tips and strategies, then check out the videos on the left, and i will see you next time. \n<\/em><\/p>\n<\/blockquote>\n
want more sat study tips?<\/h2>\n take a look at some of our other useful study tips to help you prepare for the sat math section:<\/p>\n
\t<\/p>\n\ntips to crush the sat math section | video post<\/a><\/li>\nsat math practice: how to study smarter and score higher<\/a><\/li>\n5 sat math tricks every student should know<\/a><\/li>\n<\/ul>\n<\/strong><\/p>\n
happy studying! \ud83d\ude42<\/p>\n","protected":false},"excerpt":{"rendered":"
looking for a little help with the sat math section? in this video, magoosh\u2019s sat expert chris explains the distance-rate formula! you don\u2019t have to be scared of these question types anymore, once you\u2019ve got the distance-rate formula in your pocket! watch the embedded video below, or scroll down for a full video transcript. \ud83d\ude42 […]<\/p>\n","protected":false},"author":266,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[91],"tags":[],"ppma_author":[24878],"acf":[],"yoast_head":"\n
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