Answer & Explanation
B) 3:09 pm
Efficiency of P= 100/20= 5% per hour
Efficiency of Q= 100/25= 4% per hour
Efficiency of R= 100/40= 2.5% per hour
Efficiency of S=100/50= 2% per hour
Cistern filled till 10 am by P, Q and R
Thus, at 10 am pipe P,Q and R filled 30.5% of the cistern.
Rest of cistern to be filled = 100 - 30.5 = 69.5%
Now, the time taken by P,Q,R and S together to fill the remaining capacity of the cistern
= 69.5 / (5+4+2.5+2) = 5 Hours and 9 minutes(approx).
Therefore, total time =4 hrs + 5hrs 9 mins = 9 hrs and 9 mins
It means cistern will be filled up at 3:09 pm