You and your brother went to dinner at pasta palace. you ordered a meal for $6.35: your brother ordered a meal for $7.50: the sales tax rate was 7.25% and you left a 15% tip. how much was the bill?

So you and your brother went for dinner, your meals cost is 6.35+7.50
now, here cometh the taxman, that's 13.85, and the tax on that is 7.25%

how much is 7.25% of 13.85? well, if we take 13.85 as the 100%
then  

solve for "x"

now, that's how much 7.25% is of 13.85
so, the bill that was handed out to you both was 13.85 + x

what about the 15% tip?  the tip is not part of the bill