Answer: $8,000
Step-by-step explanation: Frank’s weekly salary is calculated by the formula 400 + .15s. If he wants to earn $1,600 then it will be $1,600 = 400 + .15s.
In order to solve this you first subtract 400 from each side:
1,200 = .15s
Next, divide both sides by .15:
$8,000 = s
Frank will need to have $8,000 in sales in order to earn $1,600.