Two blocks of masses 4kg and 2kg are placed side by side on a smooth horizontal surface as shown in the figure. A horizontal force of 20 N is applied on 4 kg block. Find the normal reaction between two blocks.

The free body diagram of both the blocks are as shown in Fig. 

Using

$\Sigma F_x=m{a}_x$

For $4 \mathrm{kg}$ block,

20-N =4 a= $4\times \frac{10}{3}$

$N=20-\frac{40}{3}=\frac{20}{3}\text{ newton }$

This can also be solved as under

For $2 \mathrm{kg}$ block,

N=2 a=$2\times \frac{10}{3}=\frac{20}{3}\text{ newton }$

Here, N is the normal reaction between the two blocks.