The sale is "buy two dresses and pay the second one 50% off"
This means that she will pay full price for the first one.
And the second one will have its price cut in a half.
If both dresses costs $50.00
She will pay full price for the first one, that is $50.00
And half price for the second one, that is $50.00/2=$25.00
Without the sale she pays full price for both, that is $50.00+ $50.00=$100.00
With the sale she pays $50.00+$25.00=$75.00
$100.00-$75.00=$25.00
So if she buys the dresses during the sale, she will save $25