What is the minimum number of trucks required so that there will be no pending order at the end of the 5th day.

Question

A transporter receives the same number of orders each day. currently, he has some pending orders (backlog) to be shipped. If he uses 7 trucks, then at the end of the 4^th day he can clear all the orders. Alternatively, if he uses only 3 trucks, then all the orders are cleared at the end of the 10^th day. What is the minimum number of trucks required so that there will be no pending order at the end of the 5^th day.                                                                                                                                                         

      (a) 4                                                                    (b) 5

      (c) 6                                                                    (d) 7