Question

a complete a work alone in 10 day, B completes the same work alone in 12 days and C completes the same work alone in 15 days .In how many days they do in all working together ?

Answer 1

1/a=1/10 1/b=1/12 1/c=1/15
1/a+1/b+1/c
1/10+1/12+1/15
15/60
1/4
ans:4 days