v = 0 m/s

u = 10 m/s

s = 20 m.

v^2 = u^2 + 2as

Or, 0 = 100 + 40a

Or, a = - 100 /40 = - 5/2.

.°. a = - 5/2 m/s^2

___________

Second case,

v = 0 m/s

u = 20 m/s

a = - 5/2 m/s^2

v^2 = u^2 + 2as

Or, 0 = 400 - 5s

Or, s = 400/5 = 80.

.°. The stopping distance will be 80 m.

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