Two grain-collecting machines can collect all the grain from a field 9 days faster than if the first one was doing it alone and 4 days faster than if the second one was working alone. how long does it take each grain-collecting machine to collect all the grain by itself? last problem on my homework, all is appreciated, even an equation .

Let's define variables:
 f = number of days for the first machine
 s = number of days for the second machine
 "Two grain-collecting machines can collect all the grain from a field 9 days faster than if the first one was doing it alone":
 f-1 / ((1 / f) + (1 / s)) = 9
 "and 4 days faster than if the second one was working alone":
 s-1 / ((1 / f) + (1 / s)) = 4
 We have two equations with two unknowns.
 Rewriting the equations, you can reach the following expression:
 s ^ 2 - 8s - 20 = 0
 Factor:
 (s-10) (s + 2) = 0
 We ignore the negative root.
 s = 10
 Note: Verify that s = 10 satisfies both equations.
 
 It will take 10 days for the second machine to collect all the grain by itself
 It will take 10 + 5 = 15 days for the first machine to collect all the grain by itself.

The correct answer is c