7% of 18 = (7/100)*18 = 0.07*18 = 1.26
The amount of sales tax to be paid is $1.26
Add this onto the original amount of $18 to get 18+1.26 = 19.26
So she will have enough money (since 19.26 is smaller than 20). The amount of change Torri will get is 74 cents (20-19.26 = 0.74 dollars = 74 cents)
