Aptitude - Time and Work - Discussion

A can do 1/20 of the work per day.
B can do 1/30 of the work per day.
C can do 1/60 of the work per day.

Together, they can do 1/20 + 1/30 + 1/60 of the work per day. But since B and C only help every third day, they can do, on average, 1/20 + 1/3 (1/30 + 1/60) of the work per day.

= 1/20 + 1/3 (1/30 + 1/60).
= 1/20 + 1/3 (3/60).
= 1/20 + 1/3 (1/20).
= 1/20 + 1/60.
= 4/60.
= 1/15.

So if they can do 1/15 of the work per day, they can finish the job in 15 days.

Answer: 15 days.

I think @Manoj made it simpler and direct. Get the portion of the job that would be completed in 3 days and then keep adding that portion in 3 days interval until you get 1.

I can't understand that how the total work is calculated which is done by A.

A work in 3 days 3/20 part of the work.
Third day B and C work (1/30 + 1/60) = 1/20 part.

So, after 3 days work done by A, B and C = (3/20 + 1/20) = 1/5 part.

Now, 1/5 part done in 3 days.
1 part done in 5/1*3= 15 days (Ans).

I am Neha sharma I want to know,

How comes 6/60 = 1/10.
And 1/10 + 1/10 = 1/5.

If in 3 days work = 1/5 then how is that in 1 day =1/15.
Is it should be in 1 day 1/5 and by adding in 3 days =1/15.

Please tell me I gonne be confused.

Its very easy.

If we cancel 6/60 = 1/10.

And 1/10 + 1/10 = 2/10 (because upper term is add and lower term is common, so when we cancel 2/10 then you got = 1/5.

Guys I want know that in last step why did the multiplied 5 and three. Please explain.

Why are we multiplying it by 5 in the end?

1work/5days = 3 days or 1/5=3 then cross multiply to get 15.

Work done by A+B+C for 1 day is 1/10.

So work done by them for 3 days should 1/10 + 1/10 + 1/10 = 3/10.

How come its just for 2 days giving 1/5.