Answer:
$29,400
Step-by-step explanation:
21 years divided by 7 years = 3, which means we can separate this into 3 categories
$600 for 7 years is 4200
$1200 for 7 years is 8400
$2400 for 7 years is 16800
add the answers together
4200 + 8400 + 16800 = 29400
