For 13ths, multiply your base number by 77, subtract 1, and concatenate the number that makes it up to 999. So, for 4/13: * multiply 4 by 77 to get 308 * sub

For 13ths, multiply your base number by 77, subtract 1, and concatenate the number that makes it up to 999. So, for 4/13: * multiply 4 by 77 to get 308 * subtract 1 to get 307 * concatenate 692 to get 307692 4/13 is 0.[307692] recurring. (This trick works largely because 77 × 13 = 1001). Seventeenths are trickier, but you can get a decent approximation if you multiply the numerator by 6/100, because 1/17 is 6/102.