Press A takes 10 hours to do a certain job. So Press A does (1/10)th of the job in an hour.
Press A and Press B, together do (1/2.5) or (4/10)th of the job in an hour.
So Press B does (4/10)-(1/10) = (3/10)th of the job in an hour. So Press B can do the job in (10/3) hours or 3 hour and 20 mins.
Check: A does (1/10)th and B does (3/10)th part of the job in an hour. A and B together will take (1/10)+(3/10) = 4/10th of the job in an hour. So they will complete the job in 10/4 = 2.5 hours. Correct.
Answer: Press B can do the job in 3 hour and 20 mins.
