Another easy way to do this is a trick with multiplying numbers between 10 and 19.
Add the 2nd digit of smaller number to first and add a 0 at end, so 17+3 = 20+0 = 200
Add the product of the two 2nd digits, so 7*3 = 21 = 221
Anonymous
7 Feb 2011
15 is exactly between 17 and 13. So you can do (15 + 2)*(15 - 2) = difference of two squares = 225 - 4 = 221.