(a) A boy runs in a line and his displacement at time t seconds after leaving the start point O is X metres, where 20X = 4t\(^2\) + t\(^3\). Find the:

(i) velocity of the body when t = 15 seconds (ii) value of t for which the acceleration of the body is 8 times his initial acceleration

(b) A body of mass 6 kg moves with a velocity of 7 ms\(^{-1}\). It collides with a second body moving in the opposite direction with a velocity of 5 ms\(^{-1}\). After collision, the two bodies move together with a velocity of 4 ms\(^{-1}\). Find the mass of the second body.