What if you don't want to divide?

Divide by:	Multiply by:
1	100
2	50
3	33
4	25
5	20
6	17
7	14
8	13
9	11
10	10

For numbers greater than 10, reverse the columns and take the closest number (so to divide by 15, multiply by 7). After your multiplication you just leave out the last two digits (for more accuracy, add 1 if this number is greater than 49), and you should be relatively close (probably off by 1, or 2).