A can do a piece of work in 80 days. He work at it for 10 days and then B alone finished the remaining work in 42 days

First data: A do piece of work 80 days and

Second data: A do work in 10 days then B alone finish the remaining work in 42 days

Third question: if they are working together in how many days they finish it.

Answer:

A is One days work =1/80

A do in work in 10 days =10*1/80=1/8

Consider 1 is complete work

So reaming work is =1-1/18=7/8

7/8 work done by B in 42 days

So now we must fine B is one day work

Days* One day work= Total work done by person

One day work = Total work done by person/Days

B One day work=7/8*1/42

If you solve the equation B is one day work =1/48

If A and B is one day work if they working together=1/80+1/48=8/240=1/30

There one day work =1/30

So there finish work in 30 days source form From R.S Aggarwal Book