A boy and a block, both of same mass m kg, are initially at the same horizontal level and attached to a frictionless pulley through a light inextensible string that moves over the frictionless pulley as shown in figure. The boy starts moving upward with an acceleration of 5 m/s2 relative to the rope. If the block has to travel a total distance of 10 m before reaching at the level of pulley, the time taken by the block in doing so is equal to