Mgccl's Ivory Towerso tired

I had a dream a few nights ago, where I was asked the problem 100/6 to the nearest integer. (I was buying moon cakes, 300 for 50 dollars. Yeah, very strange dream)
It was 17. In the dream there was no calculation at all, the answer just popped up.
I woke up, and did the calculation, it was correct.
So where does 17 come from? Did I calculated it subconsciously? I didn't know I can do subconscious calculations. Maybe it's my subconscious memory of some calculation I did a long long time ago.

It could be just a random guess, because I'm sure the answer is between 10 and 20, and 11% chance getting it right.

There are instances when I go to sleep, and dream of more complex problems and solve it. I usually feels like I solved the problem in the dream. When I wake up, I can only remember the important insight. There are times where those insights are correct, and helps me solve the problem.

Do I do computation and math while I'm asleep? Or are those just random guesses that's incredibly lucky?

I wonder if there are books related to this. Google search like computation sleep doesn't help.