If all of the items have been reduced by 15%, then they are all 85% of their regular price. So that means we know that $84.15 is 85% of the regular price of that shirt.
We can express 85% as the fraction 85/100. And we can says that this fraction is equal to 84.15/x. That is because 84.15 is to x as 85 is to 100.
So we have set up the following equation:
84.15/x = 85/100
The way to solve this kind of equation is to cross multiply. This gives us
8415 = 85x
Then we divide both sides by 85 and we get
x = 99
So her shirt's original price was $99.