Answer: $178.50
Explanation:
If Alex is paid a piecework rate of $8.50 per motor, then;
Price per motor assembly = $8.50
By assembling 21 motors Alex gets,
1 motor = $8.50
21 motors = (21 * $8.50) / 1
= $178.50 / 1
= $178.50
By assembling 21 motors,
Alex gets $178.50