I use this kind of equation all the time! (In AP chem)
So if you didn't know the equation for percent error is:
|Theoretical Value - Experimental| x 100 = Percent Error
Theoretical Value
Your values for ths equation are:
Theoretical Value = 60 (since she thinks she'll spend 60 dollars)
Experimental Value = 70 (she actually spends this amount)
(The two Lines between the theoretical value and experimental value mean the absolute value of the difference.)
You have the brainpower and materials to solve this.