Aptitude - Time and Work - Discussion

We can solve it in this way also:


In 1 day, A alone can do 1/20th f the work,
So, in 2 days A can do (1/20 * 2)= 1/10th f the work
On 3rd day A,B and C together do= (1/20 + 1/30 + 1/60)= 6/60= 1/10
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Now, after 3 days, total work done= (1/10 + 1/10)=2/10=1/5
so, in 3 days they can complete 1/5th of the work.
so, for completing the whole work, they need (5*3) days= 15days

We can consider the avg of the 3 days and calculate for total work only if we know the number of days of their work is a multiple of 3. I mean if they had to work for 14 days, b and c would have assisted them only till day 12. On day 13 and 14 again a would have to work alone.

So how can we take 1/5th of work is done in 3 days?

@Yashraj

When we want to do (1/20)+(1/30)+(1/60) generally, we take the help of L.C.M, But in above case Highest number is 60 which is Divisible by all the remaining number i.e. 20 and 30.

Hence we take L.C.M. as 60 i.e. (3+2+1)/60 becomes 6/60.

Build your Basic maths Strong My friend.

(1/20) + (1/30) + (1/60) = 6/60. Please explain me this step.

I tell that third day alone A join them so on third day it is 1/20 +1/30+1/60=1/10(here a,b,c all work together)
then 2 days A never join them then B and C work together
then SO 1/20 +1/30=1/5
then make it ulta 10+5=15days

easy na.

Answer explained:

A is working for 2 days alone so A's two day work is first calculated as
A's 2 day work is =1/20*2 = 1/10
Now B and C work with A for only one day(ie 3rd day) and A also working along with B and C in the same 3rd day so A,B and C one day work(ie 3rd day work) is calculated as=(1/20+1/30+1/60)=1/5

Now A's 2 day work+ ABC one day work(ie 3rd day work is calculated)= (1/10+1/10)=1/5
Therefore in 3 days only 1/5 of work is completed
Thus to complete full work 3*5=15 days
(3days=1/5 of full wok completed
3*5 days=full work completed
15 days = full work completed)

1st day A's + 2nd day A's work = (1/20 + 1/20) = 2/20 = 1/10--(1)

3rd day work is added by B's and C's work so, work done by A+B+C=

(1/20+1/30+1/60) = 1/10--(2)

from (1) and (2)= 1/10+1/10 =1/10

Here 15 days is required for the completion of the work by A, B, C. But in question it is asked 'in how many days can A do the work' - which means the number of days A worked. So to get this we need to divide the 15 by 3 and get '5' as answer.

Can anyone please make me clear if my interpretation is incorrect ?

@Poornima

A works for 2 days and B,C joins him on 3rd day.
so total combination of them is now 3days
Based on these 3days we can furthur calculate..ok??
A's 2 days work=1/10 and on 3rd day A,B,C together work=1/10
So sum of them gives u total 3days of work done by all of them.

How can you taking that 3 days? and 1/10+1/10 what is it ?