Question

Two men can do a peace of work in six hours and four hours respectively after the first has worked four two hours he is joined by the other by when should the work be completed

Answer 1

speed of first man: 1/6 piece of work per hr
speed of second man:1/4 piece of work per hr
  Let t be the time taken by the two men
after the first man is joined by the 2nd man
2 x 1/6 + t x 1/4 + t x 1/6
= 1 1/3 + t (1/4 + 1/6)
 = 1 t
 = 1.6 hrs Total time taken
= 2 + 1.6 = 3.6 hrs