25 outlets working 6 hours a day, can empty a reservoir in 10 days. If only 15 outlets are operational and work for 4 hours a day, in how many days the reservoir can be emptied?

25 days

M1= 25 outlets, D1=10 days, T1= 6 hours/day, W2 = to fill the reservoir

M2= 15 outlets, D2=?,  T2 = 4 hours/day, W1= to fill the reservoir

W1=W2

So, the formulae is:

M1 * D1 * T1 = M2 * D2 * T2

25 * 10 * 6 = 15 * D2 * 4

1500 = 60 * D2

D2=25 Days