số tiền phải trả là
50 - 50 x 30% = 35 đô

20% off means that the new price of the skirt will be 80% of the original price:
$30(100% – 20%) = $30(80%)
Converting the percent to a decimal gives:
$30(0.8) = $24.00
There is an additional 15% off the sale price of $24.00, so the final price is 85% of the sale price:
$24(100% – 15%) = $24(85%)
Again converting the percent to a decimal gives:
$24(0.85) = $20.40