Answer : Height, h = 20.4 m
Explanation :
It is given that,
Mass of an object, m = 500 g = 0.5 kg
Gravitational potential energy, PE = 100 J
The Gravitational potential energy is the energy which is possessed due to the height and gravity of an object. It is given as :
PE = m g h
where,
h is the height of the cliff.

h = 20.40 m
So, the height of the cliff is 20.4 m.
Helium has an atomic mass of 4.00 atomic mass units.
Explanation:
Move the decimal point until it is behind the first non-zero digit.
In this case, we move it 3 places to the right so it is behind the 9.
Therefore, 0.0097 = 9.7×10⁻³.