math help fast please.?

Question

there are 2820 cars in farmington. the ratio of intermediates to compacts is 7:5 and the ratio of compacts to full-size cars is 8:9. how many full-size cars are there?

Answer 1

say there are x compacts.
then there are 7/5 x intermediates and 9/8 x full-size cars.

x + 7x/5 + 9x/8 = 2820
multiply by 40
40x + 56x + 45x = 112800
141x = 112800
x = 800
9x/8 = 900 full-size cars

Answer 2

the ratio for all three are
56:40:45
2820/141=20
1120:800:900
there are 900 full-size cars.