Newton's second law,

$\overline{){\mathbf{\Sigma F}}{\mathbf{=}}{\mathbf{m}}{\mathbf{a}}}$

Block A applies/transmits a force F_{B} to block B. Sum of forces on block B:

ΣF_{B} = F_{B} = m_{B}a_{B}

Block A experiences the applied force and reaction force from Block B. The reaction force from block B is equal to F_{B} directed opposite to the applied force.

ΣF_{A} = F_{appied} - F_{B} = m_{A}a_{A}

F_{applied} = F_{B} + m_{A}a_{A} = m_{B}a_{B} + m_{A}a_{A}

a_{A} = a_{B} = a

3. Two blocks (A and B) are in contact on a horizontal frictionless surface. A 40 N constant horizontal force is applied to A. The mass of A is 3.0 kg and the mass of B is 12 kg. What is the magnitude of the force of A on B?