Speed of a particle at 3rd and 8th second are $20 {\mathrm{ms}}^{-1}$and zero respectively, then average acceleration between 3rd and 8th second will be  [JIPMER 2019]

Time interval between 8th and 3rd second, 

                          $\Delta t = 8 - 3 = 5 \mathrm{s}, \text{ i.e. }\Delta t = 5 \mathrm{s}$

Change in velocity,

                         $∆\mathrm{v} = 20 - 0 = 20 {\mathrm{ms}}^{-1}$

$\therefore$  Average acceleration $=\frac{∆\mathrm{v}}{∆\mathrm{t}} = \frac{20}{5} = 4 {\mathrm{ms}}^{-2}$