RRB ALP CBT I Exam 9-Aug-2018 Shift 2 Solved Paper

In a computer game, a builder can build a wall in ten hours while a destroyer can demolish such a wall completely in fourteen hours. Both the builder and the destroyer were initially set to work together on the ground level, but, after 7 hours, the destroyer was taken out. What was the total time (in hours) taken to build the wall?