Aptitude - Time and Work - Discussion

@All.

Why (A+B)'s 4 days of work is calculated?
Why it's not 3,5,6,7 or any other day?
What is the logic behind it?

How come 7/60? Please explain.

A = 1/15
B = 1/20
(A+B) = 1/15+1/20 = 7/15
The remaining work = 1-7/15 = 8/15.

@Ayansh.

For this;

(A+B) 's work rate = 1/15 + 1/20 = (4/60 + 3/60) = 7/60.

A's 1 day's work = 1/15;
B's 1 day's work = 1/20;

(A + B)'s 1 day's work = (1/15+1/20)= 7/60;

Total days required to complete total work of (A+B) = 60/7,
After 4 days, remaining days left = 60/7 - 4 = 32/7,
Hence, remaining work = 32/7 x 7/60 = 8/15,
Hope it helps.

A completed this in 15 days.
Then, A completed this work in 1 days = (1/15).

B completed this work in 20 days
Then, B completed this work in 1 days = (1/20).

Both A and B both completed this work in 1 days = (A+B)
= ((1/15)+(1/20)).
then, the LCM of 15 and 20 is 60;

we get,
((1*4)+(1*3)/60))
= 7/60.

A and B both completed the same work in 4 days = ((7/60)*4)
= 7/15.

Let's take total work is = 1.
then we know that,

Rest work= total work - completed work.
= 1 - (7/15).
= (15-7)/15.
= 8/15 Answer.

Just take LCM of 15 and 20 you will get total work of 60.

Please explain, why the remaining work is 1.

@all
(1/15+1/20) =7/60.
Solution:
We have to use the mixed fraction method in a simple way using cross multiplication so it becomes (15+20)/300.
The reason for the 300 came we use the denominator direct multiplication (15*20=300)
Then 15+20 = 35,
Divide:35/300.
It becomes 7/60.

@All.

As 1 is the whole work, so when we subtract 7/15 from 1.
Then,
LCM of 1 and 15 will be 15 ( in 1 the denominator will be 1 ).
So, 15-7/15 is 8/15.