Two boxes, mass 4.0 kg and 5.0 kg are set next one another on a frictionless surface. The first box is pushed with a force of 35 N. What normal forces does the second box exert on the first box during this process? (ans: 19 N)

How do you do this question? Thanks.

a=F/(m1+m2) =35/(4+5) = 3.9 m/s²

F= m2•a =5 • 3.9 =19.5 N