Ok - this may not help much, but here goes:
A car company manufactures 10 cars at a cost of $7 per car. (thus x=10 and c=7)
If some cars are sold below cost at $4 per car, then the rest are sold at normal retail price of $9 per car. (s=4, n=9)
Total cost is 70
Revenue is 4*(#at sale price) + 9*(# at retail price)
# at sale price is A and # at retail price is 10-A. We want to solve for A here, so i must leave it a variable instead of turning it into a number.
4A + 9(10-A) = 70
4A + 90 - 9A = 70
-5A = -20
5A = 2-
Then, we look at which of the answer choices will give us A=4.
A. 10(7-9) / 4-9 = -20 / -5 = 4 --> might be our right answer
B. 10(9-7) / 4-9 = 20/-5 = -4 --> can't be our right answer
C. 10(7-9) / 4-7 = -20/-3 --> can't be our right answer
D. 10(4-9) / 7-9 = -50 / -2 = 25 --> can't be our right answer
E. (10-9)(10-4) = 1*6 = 6 --> can't be our right answer.
Obviously, this is a much longer and more complicated way to approach the problem, but this is how it would work if you were to approach it this way.