Question 691734
1 pump can empty a tank in 8hr. and 15 min=8+15/60 hr=33/4 hrs
so in 1 hr it will empty 4/33 parts
another pump can empty the same tank in 11 hr 
so in 1 hr it will empty 1/11 parts
with both pumps running at the same time
in 1 hr both can empty 4/33+1/11=7/33 parts
so the time taken is 33/7 hrs=4 hrs 43 min approx to empty the same tank with both pumps running at the same time.