Internal forces can change

$A$ rope ladder with a length $l$ carrying a man with a mass $m$ at its end is attached to the basket of a balloon with a mass $M$. The entire system is in equilibrium in the air. As the man climbs up to the end of the ladder into the balloon,the balloon descends by a height $h$. Then the potential energy of the man:

Why are the internal forces acting on the centre of mass of a system neglected?

State Newton's second law for a system of particles.

$Assertion$ : If no external force acts on a system of particles,then the centre of mass will not move in any direction.
$Reason$ : If net external force is zero,then the linear momentum of the system changes.

If a force $10 \hat{i} + 15 \hat{j} + 25 \hat{k}$ acts on a system and gives an acceleration $2 \hat{i} + 3 \hat{j} - 5 \hat{k}$ to the centre of mass of the system,the mass of the system is