Is it normal for a 20 year old woman to sleep an average of 10 hours a day?

Could be.: There are the so-called "short sleepers" who need less than 5 hours of nightly sleep to function normally. There are the so-called "long sleepers" who need more than 10 hours of sleep. Both are considered variants of the normal. Most of adults will be somewhere in between: needing 5-10 hours of sleep. Hope, this 20 year old functions normally during the day after 10 hours of sleep.

Can be: The average person should sleep 8 hours. Since its an average, some people sleep less and some more. As long as you feel well and refreshed after you wake up, then it's probably fine.