Go here to review percents if you're feeling a little fuzzy about them. If you're still feeling fuzzy after reviewing that section, you should see a doctor. You may have a concussion. Hopefully, it wasn't algebra-induced.
15% of what number is 6?
This is a fairly straightforward question, so the first thing we need to do is to make sure we're interpreting it correctly. We're not looking for 15% of 6. Instead, we're looking for a number that will give us 6 when we take 15% of it. In this example, "what number'' translates to x, and "15% of'' means "0.15 times." If you take an algebra test and you get it back with a red "0.15" scrawled across the top, you'll know how well you did. Answer: not so well.
Now we solve the equation
0.15x = 6
and get x = 40. We can backtrack and double-check our work to make sure that's right. Sure enough, 15% of 40 is 6. Hot diggity.