A body has mass 400kg on earth. What will be it mass on the moon if the Acceleration due to gravity is 9.8m/s2?

1. The mass in kg is mass PERIOD EVERYWHERE

2. The weight, or in other words gravitational FORCE is the mass times the local acceleration of gravity:
F = W = m g

on earth mg= 400*9.8=3920 Newtons

on the moon g is about 1/6 g on earth so
on moon mg = (1/6)*9.8*400=653 N