You're expected to know that: .
We have: .A body moves in such a way that its position at time is given by: . , where n is a constant.
(a) Find when the velocity is doubled from the 5th to the 10th second.
We have: .(b) Find when the velocity is numerically equal to the acceleration at the end of the 10th second.
These two quantities are equal: .
. . . . . Therefore: .