What’s the dependent variable

Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much

Gabriel determined that his total cost would be represented by 2.5x + 2y – 2. his sister states that the expression should be x + x + 0.5x + y + y – 2. who is correct? explain.

What adds to be the bottom number but also multiplies to be the top number with 8 on the bottom and 15 on top