Answer:
7%
Step-by-step explanation:
You want the sales tax rate that makes a $78 phone end up costing the customer $83.46.
Tax at rate r causes the cost to be multiplied by 1+r:
78(1+r) = 83.46 . . . . . . . . . . . . . effect of tax
1 +r = 83.46/78 = 1.07 . . . . . . . divide by 78
r = 1.07 -1 = 0.07 = 7% . . . . . . subtract 1
The sales tax percentage is 7%.