Question

To do a certain job A takes three times as long as the B takes.If both of them working together take nine days how many days will A take to finish the job working alone?
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Answer 1

Let us suppose the no. of days A will take to finish the work be X.
Then work done by A in one day will be 1/X
But A takes 3 times longer than B                         (given)
That means, work done by B in one day will be 
1/X multiplied by 3 which is 3/X
now, the total work done by both A and B in one day will be-
1/X+3/X = 4/X
they take 9 days to complete the work, which means the total quantity of the work is -
4/X multiplied by 9 = 36/X 
now, we know that A can do 1/X work in one day and the total work is 36/X, so for calculating the no. of days taken by him, we divide the total work by his one day's work --
36/X divided by 1/X 
which will be 
36/X * X/1
= 36 
so, A will take 36 days to finish the work.