A body of mass 10kg is moving with an initial speed of 20m s ^ - 1...

Friction force wil act opposite to the motion of the body.

$F = \muN = \mumg$

$⇒ a = - \mug\left(\text{-ve sign for direction relative to the motion of body}\right)$

Now, applying equation of motion, we get

$v = u + at$

$⇒ 0 = 20 + \left(- \mu \times 10\right) \times 5$

$⇒ 50\mu = 20$

$⇒ \mu = 0 . 4$