Quick thing How did they get this?

[neutron star]

https://www.physicsforums.com/showthread.php?t=259689

In that thread how did the person figure out the contact force between mA and mB? I know they got the acceleration by doing F/mA+mB. But I don't get how they got the contact force, because all the blocks are equal mass. Something like F-a*(10+10/2)? What are they multiplying the F-a by?

Thanks.

 

[rl.bhat]

In the problem applied force is given, masses are given. Find the acceleration.
a = F/3m.
For the time being, forget the first mass.
How much force is to be applied to the remaining two masses to have the acceleration a?
That is the contact force acted by the first mass on the remaining two masses.