Add the amounts for the first 7 cars:
10 + 12+ 15 + 15 +15 +9 + 10 = $86
The mean is the average, if the mean is $12, multiply that by the total cars (8):
12 x 8 = $96
This means for 8 cars he earned $96.
To find the amount he made for the 8th car, subtract the total he earned from the 7 cars from the total he made for all 8:
96 - 86 = 10
He earned $10 for the 8th car.
