SAT Mathematics Practice Test 3

A system of equations can be used where h represents the number of calories in a hamburger and f represents the number of calories in an order of fries.
The equation $2\mathit{h}+3\mathit{f}=1700$ represents the fact that 2 hamburgers and 3 orders of fries contain a total of 1700 calories, and the equation $\mathit{h}=\mathit{f}+50$ represents the fact that one hamburger contains 50 more calories than an order of fries.
Substituting f+50 for h in $2\mathit{h}+3\mathit{f}=1700$ gives $2\left(\mathit{f}+50\right)+3\mathit{f}=1700$.
This equation can be solved as follows
$2\mathit{f}+100+3\mathit{f}=1700$
$5\mathit{f}+100=1700$
$5\mathit{f}=1600$
$\mathit{f}=320$
The number of calories in an order of fries is 320, so the number of calories in a hamburger is 50 more than 320, or 370.