Proportions can help you calculate discounts and sales tax by comparing the percentage with the original price. For example, to find a 20% discount on a $50 item, you can write 20/100 = x/50 and solve for the discount amount, which is $10. The same method can be used to find sales tax or the final price after adding or subtracting a percentage. A proportion solver makes these calculations easier by finding the missing value and showing the steps, which can help you check your answer and avoid simple mistakes.