video explanation

text explanation

first, we'll convert those statement to equations.  let l be the price of one lemonade and c the price of one cookie.  

5l + 2c = 1.50

2l + 5c = 2.70

of course, we could solve one equation for one variable and substitute that into the other; that would allow us to find the value of one variable, and then we could find the value of the other.  that would be the long way to solve the problem.  

instead, when the test asks you to find a combination, such as l + c, often there's a way to get to that directly without solving for the individual variables.  

here, notice that the coefficients in the second equation are the same as in the first, just in a different order.  in other words, if we add the two equations, both variables will have the same coefficients: 

7l + 7c = 4.20

getting the same coefficients on l and c is useful.  now, we simply can divide this equation by 7: 

l + c = 0.60

that's what the question is asking.  the price of one lemonade and one cookie is 60¢. 

answer = (c)