Find the HCF of the fractions 4/9, 10/21 and 14/15.

HCF of fractions = HCF of the numerators / LCM of the denominators. HCF(4, 10, 14) = 2 and LCM(9, 21, 15) = $3^2\times 5\times 7=315$, so the answer is 2/315. Swapping the two rules gives 140/3, which is in fact the LCM of the three fractions; 2/45 uses only the denominators 9 and 15; 14/315 takes the largest numerator instead of the HCF of the numerators.