A motorcycle is being driven at a speed of $20\ m/s$ when brakes are applied to bring it to rest in five seconds. The deceleration produced in this case will be:
(a).\ +4\ m/s^2$
(b).\ -4\ m/s^2$
(c).\ +0.25\ m/s^2$
(d).\ -0.25\ m/s^2$

Here given, initial velocity $u=20\ m/s$

Final velocity $v=0$

Time $t=5\ s$

Therefore, deceleration $=-$acceleration

$=-(\frac{v-u}{t})$

$=-(\frac{0-20}{5})$

$=-(-4\ m/s)

$=+4\ m/s$

So, option (a) is correct.