Aptitude - Time and Work - Discussion

How 7/60 came? Please explain.

Thanks for explaining the answer @Anjali thakur.

@L Raj Scope

Please understand this concept first.

LET CONSIDER A SIMPLE STATEMENT:

A can do a work in 2 days.

That means A's 1 day work = 1/2.

This is the methodology we wanna apply on the data which is given in question.

From the question.

A (CAN D0 A JOB) ----15 days.
B (CAN DO A JOB) ----20 days.

Therefore by the above concept A's 1 day work = 1/15.

B's 1 day work = 1/20.

Therefore both A&B 1 work = (1/15)+1/20) = 7/60.

NOW READ THE QUESTION.

If A&B work together for 4 days then what the fraction of work left.

Consider:

Total work = 1.

A&B (4 DAYS WORK) = (7/60)*4 = 7/15.

REMAIN B WORK = (TOTAL WORK)-(COMPLETED WORK BY A&B FOR 4 DAYS).

= 1-7/15.

= 8/15.

Hope this is helpful.

LCM of A's and B's:
A-15
B-20.

LCM is 60 (It is also total Work) and Efficiency is TOTAL WORK/ TIME i.e.
60/15 = 4 <<<A's 1 day Efficiency
60/20 = 3 <<<B's 1 day Efficiency.

Combining both 1-day Efficiency then we get 4+3=7 (It means A and B can do 7 units in 1 day)

According to the question:
if they have 7 unit Efficiency they work for 4 days then we get:

7 x 4 and divide by total work (60).
7 x 4/60= 7/15 << it is the part of work they will finish in 4 days.

But the question was asked about how much fraction is left then simply we know that total work is 1 then subtract total work 1-7/15.

We get 8/15 that is the answer.

How 1-7/15? Explain.

I am not getting. How 1-7/15? Explain, please.

Thanks everyone for explaining the answer.

1/15+1/20 LCM of this is 60.
So,
(1/15 * 60 + 1/20 * 60)/60we can simplify this you can get (4+3)/60 = 7/60.

(+1-7/15).
(-*-=+) = 8/15.

Agree, the answer is D.