Final answer:
Elsa spent $15 on a new outfit and used $15 of her savings to buy a sweater. The part of her savings that Elsa spent on the sweater is a) ${1}/{3}.
Step-by-step explanation:
To find the part of her savings that Elsa spent on the sweater, we need to calculate the ratio of the amount spent on the sweater to the total savings. Elsa spent $15 on the sweater, and her total savings were $15. So, the ratio is $15 / $15.
By simplifying the fraction, we find that the part of her savings Elsa spent on the sweater is 1/1, which is equal to 1.
Therefore, the answer is a) ${1}/{3}.