They had 17/12 ( 17 over 12, or 1 and 5 over 12) So the fractions have to have the same denominator. 12 is their common denominator. So: 3/4 = 9/12 2/3=8/12
In writing 12 less than a number x, the expression, 12 - x is wrong because "less than" is not the same thing as "minus" (-). Rather, the sign, "<" should have been used as follows: