Work done by A in 10 days =(1/80*10)=1/8
Remaining work = (1- 1/8) =7/ 8
Now,7/ 8 work is done by B in 42 days.
Whole work will be done by B in (42 x 8/7) = 48 days.
A’s 1 day's work = 1/80 and B's 1 day's work = 1/48
(A+B)'s 1 day's work = (1/80+1/48)=8/240=1/30
Hence, both will finish the work in 30 days.