If an astronaut weighed 175 pounds on earth he would have weighed only 29 pounds on the moon if his mass on earth was 80kg what would his mass have been on the moon

Weight is measured by the force of gravity, which changes throughout different planets, moons, etc.

Mass is the measure of the amount of mass in an object, in which it never changes, no matter where you are. So if the astronaut's mass on earth was 80 kg, his mass on the moon would still be 80 kg