1. Algebra

At the grocery store, Sal buys 7 cans of beans, a jar of peanut butter and 4 cartons of milk, which costs $28.55 total. Hal buys 9 cans of beans, a jar of peanut butter and 5 cartons of milk, which costs$35.30 total. If I buy a can of beans, a jar of peanut butter and a carton of milk, i will pay:
a 8.10 b 8.15 c 8.20 d 8.25 e 8.30

this is what i have done so far.
b--> beans
p--> peanut butter
m --> milk carton

$\displaystyle 7b + 1p + 4m = 28.55$=> $\displaystyle 1p = 28.55 - 7b - 4m$
$\displaystyle 9b + 1p + 5m = 35.30$=>$\displaystyle 1p = 35.30 - 9b - 5m$

therefore
$\displaystyle 35.30 - 9b - 5m$ =$\displaystyle 28.55 - 7b - 4m$
$\displaystyle 2b + 1m = 6.75$

but i dont know what to do from here, or whether i even started right.

2. If you have 3 unknowns you need at least 3 equations.

You have 2 equations with 3 unknowns.

Do you have any further information?

3. Solve the system

$\displaystyle m=\frac{6.75}{2b}$ right?

go substitute that back in the equation

=

and solve!