Answer:
3 hours
Step-by-step explanation:
Assuming that when the problem says "day" it refers to the hours of wakeness. The problem is essentially asking to find 20% of 15.
First, convert 20% to decimal form. To do this, divide (20) by 100, this is because a percent is out of 100.
20 / 100 = 0.2
Now multiply the hours of wakeness by the percent as a decimal;
15 * 0.2 = 3
Isabelle spends 3 hours studying.
