Question 81772
Time taken by press A = 7 hrs.
So work done in 1 hr = 1/7 part

Let the time taken by press B = x hrs
So work done in 1 hr = 1/x part

Together work done in 1 hr = 1/7 + 1/x

                           = (x + 7)/7x

Given they complete in 5 hrs.
==> they complete 1/5 part in 1 hr

So (x + 7)/7x = 1/5

==> 5(x+7) = 7x

==>  5x + 35 = 7x

==> 35 = 7x - 5x
==> 35 = 2x
==> 35/2 = x
==> 17.5 = x

Thus B takes 17.5 hrs to complete all by itself