Three blocks of mass 3 kg, 4 kg and 6 kg are line up in a row on a table and a force of 18 N is applied to the block with a mass of 3 kg.
The total mass of the three blocks is 3+4+6 = 13 kg. When a force of 18 N is applied they are accelerated at 18/13 m/s^2. The force exerted by the 4 kg block on the 6 kg block is equal to the product of the mass of the block and its acceleration. This is 6*(18/13) = 108/13 N.
The force applied on the 3 kg block by the 4 kg block is the same as the force applied on the 4 kg block by the 3 kg block as action is equal to reaction. This gives the force as 4*(18/13) = 72/13 N.