First calculate all the deduction
FICA = $10 per hour x 0.0765 = $0.77 per hr
Fed tax = $10 per hr x 0.12 = $1.2 per hr
State tax = $10 per hr x 0.08 = $0.8 per hr
Remaining pay = 10 – 0.77 – 1.2 – 0.8 = $7.23 per hour
Time required = $640 / $7.23 per hr
Time required = 89 hrs