Monthly payment = $20,000
Payment per album = $12
Total earned last month = $24,800
Part (a): Expression for money earned last month
Let x be number of albums last month. Then,
Total earning = Fixed payment + Variable payment
That is,
24,800 = 20,000 + 12*x
24,800 = 20,000 + 12x
Part (b): Number of albums last month
This involves solving for x in the expression derived in part (a) above.
24,800 = 20,000 + 12x
24,800 - 20,000 = 12
12x = 4,800
x = 4,800/12 = 400 albums