Two blocks of masses $4\,\text{kg}$ and $6\,\text{kg}$ are in contact on a frictionless horizontal surface. A force of $50\,\text{N}$ pushes the $6\,\text{kg}$ block, causing both blocks to accelerate together. What is the contact force between the two blocks?

The acceleration of the system is $a = 50/(4+6) = 5\,\text{m s}^{-2}$. Now the contact force is the force required to accelerate the $4\,\text{kg}$ block, so $N = 4 \times 5 = 20\,\text{N}$. Option A is correct. A common mistake is to use the $6\,\text{kg}$ block here, which would give $30\,\text{N}$.