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 initial velocity is u=20 m/s and final velocity is  5 m/s and distance travelled is 100 m. Then from the equation

$$\begin{gathered} {\mathrm{v}}^2-{\mathrm{u}}^2=2\mathrm{as} ; 5^2-{20}^2=2\mathrm{a}\times 100 \\ \mathrm{a}=-\frac{25\times 15}{2\times 100} \\ \mathrm{and} \mathrm{F}=\mathrm{ma}=-20\times \frac{25\times 15}{2\times 100}=-37.5\mathrm{N} \end{gathered}$$