For children, adolescents and educated adults, comparing fractions with common numerators (e.g., 4/5 vs. 4/9) is more challenging than comparing fractions with common denominators (e.g., 3/4 vs. 6/4) or fractions with no common components (e.g., 5/7 vs. 6/2). Errors are related to the tendency to rely on the “greater the whole number, the greater the fraction” strategy, according to which 4/9 seems larger than 4/5 because 9 is larger than 5. We aimed to determine whether the ability of adolescents and educated adults to compare fractions with common numerators was rooted in part in their ability to inhibit the use of this misleading strategy by adapting the negative priming paradigm. We found that participants were slower to compare the magnitude of two fractions with common denominators after they compared the magnitude of two fractions with common numerators than after they decided which of two fractions possessed a denominator larger than the numerator. The negative priming effects reported suggest that inhibitory control is needed at all ages to avoid errors when comparing fractions with common numerators.