Step-by-step explanation:
Rob's mistake lies in his approach to dividing 120 in the ratio of 3:5. While he correctly divided 120 by 3 to get 40, and by 5 to get 24, he failed to consider the ratio itself in his calculation.
To divide a quantity in a given ratio, Rob should have first found the total parts of the ratio, which is 3 + 5 = 8. Then, he should have divided 120 by this total number of parts (8) to find the value of one part. After finding the value of one part, he could have multiplied it by the respective parts in the ratio to obtain the desired amounts for each part of the ratio.
Therefore, Rob's mistake was not considering the total parts of the ratio before dividing 120.
