Question

A and b can do a piece of work in 28 days.with the help of c they can finish it in 21 days.how long will c take to finish it

Answer 1

See it is the question of time and work you must know basic to do that but I will tell the solution because it's so much see the solution
Let 28 days of a and b be 1/28
And 21 days of b be 1/21
By adding subtracting both we get answer
1/28-1/21
After LCM
The solution will be 1/12
Means c take 12 days to complete it