Let us make sure we understand mass and weight are not the same thing. 80 kg of mass is not the same thing as 80 N of weight. To calculate weight, we use Newton's 2nd Law of Motion, which gives us the formula "Force = mass x acceleration". We would take the mass given, 80 kg, and multiply it times the acceleration of Earth's downward pull on that mass, 9.8 m/s/s.
784 N = 80 kg x 9.8 m/s/s
So the weight on Earth is 784 newtons.
Now the gravitational attraction of the moon is about 1/6 that of the Earth, or about 17%. To calculate the weight on the moon, simply multiply 784 N times .17, or 17% in decimal form:
784 N x .17 = 133.28 N
So the weight of the same 80 kg mass on the moon is 133.28 N.