Answer
You can spend up to $10.75.
Work Step by Step
Let a be the additional amount you can spend. The additional amount added to 4.25 dollars must not exceed 15 dollars.
a+4.25≤15⟶ subtract 4.25 from each side.
a+4.25−4.25≤15−4.25⟶ subtract
a≤10.75