Question

4. A pump can empty a small pond in 4 hours. Another more efficient pump can do the job in 3 hours. How long would it take to empty the pond if both pumps were used

Answer 1

one pump does 1/4th of the work in one hour
other does 1/3 rd of work in one hour
together they will do
1/4+1/3=7/12 work in one hour
so they'll complete the work in 12/7 hours or 1.714 hours