IT is called UNIFICATION method on Mathematics

Problem with Chintu and Mintu

Chintu can do a task in 24 days while Mintu can do it in 30 days. In how many days can they complete the task when work to gether?

Now let us take the complete work as 1 

Chintu can complete the work in 24 days. 
So in one day Chintu can complete 1/24 part of the work

Similarly Mintu is capable of doing the work in 30 days.
So in one day Mintu completes 1/30 parts of work.

Now when they both works  in one day total work completion  is  1/24 + 1/30 parts of work
 1/24+ 1/30 
= 54/720= 3/40 parts of the total work


Now  we can say that 

3/40 parts of work is being completed in 1 day

so 1 work(complete) will be completed in 40/3 days= 13.33 days