91/72 cannot be simplified anymore, but if you divide (commonly on a calculator), you can achieve the decimal form of the fraction which is, 1.2639 OR, 1 with a remainder of 19.
If you want the mixed number form, you will have to divide the numerator by the denominator. In this case, just use the decimal form or, 1 with a remainder of 19. Your mixed number would be 1 19/72.