Kumail, a noted sneaker enthusiast, has a collection

We'll start from the smallest possible group we know about and work our way up:

We know that 1/3 of the Brand A low-tops are running shoes. For this to work, the number of Brand A low-tops must be divisible by 3.

We know that 5/7 of the Brand A sneakers are low tops. For this to work, the number of Brand A low-tops must be divisible by 5, and the number of Brand A sneakers must be divisible by 7.

Finally we know that Kumail has 7 more pairs of Brand B sneakers than Brand A sneakers. This doesn't tell us anything about the number of Brand A sneakers we need to have.

Okay, we know that the number of Brand A low-tops must be divisible by 3 and 5. The lowest common multiple of 3 and 5 is 15. If 15 is 5/7 of the number of Brand A low-tops, then there must be 21 total Brand A sneakers. Then, Kumail must have 21 + 7 = 28 Brand B sneakers, for 21 + 28 = 49 sneakers in total.