Solve this problem in less than 2 minutes! Okay, maybe 3, um, 4??? :/
3568=3
8066=5
7601=2
8166=4
6233=1
1541=0
9888=7
6906=4
8968=6
4500=?

Nothing, huh? Come on! Don't overheat your brain. Actually, kids can solve this problem in less than 5 minutes. Ooh, that's going to be a giant bummer if you can't. Those kids will eat you alive!
P.S. This blog entry is dedicated to dear Onee-chan and anybody who's interested in solving math puzzles and/or problems.
P.P.S. Spoiler alert! I'm not interested in math or any