Question

A can complete a job in 15 days B can complete same job in 21 days If they work together how many days it will take

Answer 1

ab/a+b
15*21/36
8 days 18 hours

Answer 2

A can do the whole work in 15 days

So, in one day he can do 1/15 of the work;

B can do the whole work in 21 days

So, in one day he can do 1/21 of the work

Together in 1 day they can do : 1/15 + 1/21 of the work = 7/ 105 + 5/ 105 of the work = 12/ 105 of the work

So, the time they would take to do the whole work together would be : 1 / ( 12/105) = 105/12 =35/4 = 8 and 3/4 days; And since there are 24 hours in a day, so, 3/4 of a day would be 3/4 * 24 = 18 hours

So, answer : 8 days 18 hours

Hope it helps!

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