The best way to work this is out, is to find out how many inches there are in a foot, and half a foot. One foot is equal to 12 inches, and half a foot is equal to 6 inches. It's best to find the whole numbers first. The best way to do this is to simply multiply the length (12in) by the feet (13), 13x12= 156 inches.
You've then got half a foot, which is six inches, so you then have to add this to the 156, giving you 162.
In thirteen and a half feet, there are 162 inches,
Hope this helps :)