Aptitude - Time and Work - Discussion

A's 1 day's work = 1/15,

B's 1 day's work = 1/20,

Since they work together for 4 days so that their left work in fraction = [1 - (1/15 + 1/20)]*4.

= (8/60)*4.

= 8/15.

Short trick if you are use to of percentage:

A's 1 day's work = 6.67% (100/15).
B's 1 day's work = 5%.

A+B's 1 day's work = 11.67%.
A+B's 4 day's work = 46.68%.

Remaining... 100-46.68 = 53.33% i.e. 8/15.

@Deepa.

(1/15+1/20) L.C.M of 15, 20 is 60.

So 1/15*60 = 4.

And 1/20*60 = 3.

4+3/60 = 7/60.

Why have you taken 1 as total work? HOW?

I understood upto 7/15. I can't understand how they are telling total work 1. From that they were minusing 7/15 can you explain me.

@Jansi.

Both A and B doing the same (one) work, not different work.

We have to divide the denominator by its common divisible number and cross multiply it in numerator so you will get 7 then mutilply the highest remainder of lcm with the lowest denominator. You will get answer.

How the 0.53333 come 8/15 ?

a/b + c/d.

= a*d/b*d + c*b/b*d.

= ad+cb/b*d.

Maths rule if denominator not equal if equal directly cross multiplication.

When dealing with addition or subtraction of fractions you consider the denominators i.e
15&20 and then you find the L.C.M which is 60 or multiply them together.

15*20 = 300 so,

(1/15 + 1/20)/300 = (20 + 15)/300.

=25/300 reduce to the lowest term and you get 7/60.

This 7/60 is the amount of work A & B will complete in one day.
Hence for 4 days we have;

7/60 * 4 = 7/15 of the total work.

Since we don't know the real value of the total work we then assume total work to be done to be 1.

Therefore,

Remaining work left will be,

1- (work done)

1-7/15 ; the denominator here is 15 & 1 (since 1 =1/1).

Do the math and you get our final answer to be 8/15.

NOTE !

If you are subtracting a fraction from a whole number, just multiply the denominator by the whole number and then subtract from the numerator. The same rule applies for addition.