Brooke and two friends went to the movies and spent a total of $42. the movie tickets were $5 each and they each bought a popcorn combo. what is the cost of one popcorn combo?

Take 42 (total amount) and subtract 15 (there are 3 friends, $5 each).

Equation: 42 - 5(3) = 27.

So they spent $27 in all for popcorn. But you need to find the price for one. So take 27 and divide it by 3 since the 3 friends each bought a combo and you would get 9. 

One popcorn combo is $9. 

To check this you can add it all up 9 * 3 + 5 * 3 = 42
27 + 15 is 42 making the answer correct. I hope this helps!