The velocity of a body of mass 20 kg decreases from 20 m/s to 5 m/s in a distance of 100 m. Force on the body is 

 Here $m=20kg, u=20m/s, v=5m/s, s=100m$ 

using  $v^2=u^2+2as$

 $F=ma=mx\left(\frac{v^2-u^2}{2s}\right)$
Substituting the values, we get 

$F= 20 \times \left(\frac{{\left(5\right)}^2-{\left(20\right)}^2}{2\times 100}\right)=-37.5N$