Bunuel
Kumail, a noted sneaker enthusiast, has a collection consisting of sneakers made by Brand A and Brand B. 5/7 of the Brand A sneakers are low-tops, and 1/3 of the Brand A low-tops are running shoes. If Kumail owns 7 more pairs of Brand B sneakers than Brand A sneakers, what is the minimum possible number of pairs of sneakers in his collection?
A. 21
B. 28
C. 36
D. 40
E. 49
We are given that 5/7 of the Brand A sneakers are low-tops and 1/3 of the Brand A low-tops are running shoes. Thus, the number of Brand A sneakers must be a multiple of 7 x 3 = 21. Since the smallest positive multiple of 21 is 21, the least number of Brand A sneakers is 21.
Since Kumail owns 7 more pairs of Brand B sneakers than those of Brand A, the minimum number of Brand B sneakers is 28. Thus, the minimum possible number of pairs of sneakers in his collection is 21 + 28 = 49.
Answer: E
"Thus, the number of Brand A sneakers must be a multiple of 7 x 3 = 21." Can anyone elaborate on this, how it would be a multiple of 7 & 3?