Final answer:
Jeremy's answer is incorrect because 3/4 plus any positive fraction cannot be smaller than 3/4, and the resulting fraction must have 20 as the denominator, not 9. B. Jeremy's answer does not make sense when adding fractions.
Step-by-step explanation:
To understand why, we can use our intuition about fractions without performing the actual calculation. When adding fractions, the sum must be greater than either of the individual fractions if they are positive. As we know from basic number sense, adding something to a number makes it larger. Jeremy added 3/4 and 1/5. If we visualize this, 3/4 is already greater than 1/2, and adding any positive amount to it, even if it's small like 1/5, would result in a sum that is larger than 3/4. Therefore, the answer cannot possibly be as small as 4/9, which is less than 1/2. This proves that Jeremy’s original answer of 4/9 is not possible.
Moreover, when adding fractions with different denominators, one must find a common denominator. The denominators 4 and 5 do not have any common factors other than 1, so the smallest common denominator would be their product, which is 20. Knowing this, it's clear that the sum of 3/4 and 1/5 must have a denominator of 20, and since 4/9 does not, Jeremy's calculation must be incorrect.