Answer:
Cr = 10
Step-by-step explanation:
To calculate combinations we use the nCr formula: nCr = n! / r! * (n - r)!, where n = number of items, and r = number of items being chosen at a time.
![C(n,r)=[?]](https://tex.z-dn.net/?f=C%28n%2Cr%29%3D%5B%3F%5D)




[RevyBreeze]
Suppose the larger pump alone can empty the tank in L hours, and the smaller pump can finish the job in S hours, then each hour the large pump empties 1/L portion of the tank, and the small pump empties 1/S per hour
Working together for three hours, they empty the whole tank, which is 100% of it, so 3/L+3/S=100%=1
Larger pump can empty the tank in 4 hours less than the smaller one, so L=S-4
replace L: 3/(S-4)+3/S=1
Make the denominator the same to solve for:
3S/[S(S-4)] +3(S-4)/[S(S-4)]=1
(3S+3S-12)/[S(S-4)]=1
(3S+3S-12)=[S(S-4)]
S^2-10s+12=0
use the quadratic formula to solve for S
S is about 8.6
The answer is not whole hour.