Unknown is Cup:
3x=30
x=10
Unknown is Hamburger:
10+2x=20
2x=20-10
2x=10
x=5
Unknown is Fries:
5+2x=9
2x=9-5
2x=4
x=2
All now known, plug into final equation:
5+2*10
5+20 (order of operations dictates multiplication first)
25 is the final answer
EDIT:
Ooopppss....Just noticed that pair of fries is 2 but final equation has single fries, which must be 1....Sooo.....
5+1*10
5+10
15
Got me on that one!