Arestaurant bill comes to $28.35. find the total cost if the tax is 6.25% and a 20% tip is left on the amount before tax.

The question is asking for the total cost after a tip and tax is added to the subtotal. In this case, you will be multiplying those percentages by the subtotal and adding them together to get your answer.
Sub-total = Original pricing ($28.35)
$28.35 * 6.25% = $30.14
Multiply the total by the tip to get the final answer 
$30.14 * 20% = $36.17
Basically, you are multiplying the percentages by the number to find out how much of it that percentage is. Then you are adding that to the sub-total to find the total. You do this twice when it asks for these.