This is the deniomenator of fraction. You can pair 1to 9 to get 10, do this with every number, 5 will be in the middle with no pair, you get 4 pairs, 4*10=40, 40+5=45. For the numerator, you can do the samething, pair 11 to 20 to get 31 it does pair up perfectly, so you compute 31*10 which is the same as 310. I got 310/45= 62/9. There is also a formula, this website should help, http://mathcentral.uregina.ca/qq/database/qq.02.06/jo1.html . Not sure if I made a mistake or not. Visit the website just in case.