Brian spent a total of $78 while shopping, $22 of which he spent on tank tops.
The total spent is $78, which represents 100% of the money spent.
To determine the percentage spent on the tank tops you can use cross multiplication:
$78 → 100%
$22 → x %
Both relationships are at the same ratio so that:

Multiply by 22

He spent 28% of his purchase on tank tops.