Question

A can do a piece of work in 5 hrs. B in 9 hrs and c in 15 hrs. If c could work with them for one hour only, what is the time taken by a and b together to complete the work.

Answer 1

A work in 1 hour=1/5

B work in 1 hour =1/9

C work in 1 hour = 1/15

a+b+c work in 1 hour = 1/5 + 1/9+ 1/15=17/45

c's work in 1 hour= 1/15

after 1 hour remaining work = 1-1/15=14/15

a+b work in 1 hour= 1/5+ 1/9 = 14/45

remaining work by a+b in 1 hour = 14/45 * 15/14 = 1/3

so a and b work together to complete this work in 3 hours.