Percentage
We know that the 100% of the diagram correspond to 24 hours. We can see that Antoine sleeps 40% of the day. We want to know what is the 40% of 24h.
We have the following equivalence:
40% ⇄ ?
100% ⇄ 24h
If we divide both sides of the equivalence we will have the same result

if we multiply both sides if the equation by 24:

Answer: Antoine sleeps 9.6 hours