Initial velocity ( u ) = 100 cm / sec. Retardation ( - a ) = - 5cm / sec^2. { Retardation means negative acceleration } Time ( t ) = 15 sec. Final velocity ( V ) = ? So, we KNOW from the formula, Acceleration = (Final velocity - Initial velocity)÷ Time.i.e. a = ( v- u ) / t . -5 = ( v - 100 ) / 15 -5 × 15 = v- 100. { transposing 15 to LHS } - 75 = v - 100. -75 + 100 = v { transposing -100 to LHS } v = 25 cm/sec. Therefore , 'v' i.e. final velocity = 25cm/sec.Now, we NEED to find the time at which the body comes to rest. Here, u = 25 cm/sec. { because the body is now going to be at rest , hence the final velocity becomes the initial velocity } v = 0 cm / sec . { the body is at rest , hence no movement } . acceleration = -5cm / sec^2. Again , from the formula, a = ( v - u ) / t -5 = ( 0 - 25 ) / t. -5 = -25 / t. -5T = -25 t = -25 / -5. t = 5 sec. Hence , the body will come at rest in 5 seconds.