There are 24 hours in a day, so 9 hours of the day is 9/24.
To reduce this, you need to recognize that both 9 and 24 are divisible by 3: 9=3x3 and 24=8x3
If you divide both by 3, you’d get your answer of 3/8.
You should sleep 3/8 of the day, when you’re 10 to 17 years old.
