65-year old dummy needs help

If you have a sales receipt that shows only the total that was charged to your card, but not the breakdown of items purchased and tax charged, and you know that the tax rate is 7.5% (.075), how can you determine the total amount of the items purchased and the tax charged that added up to the amount on your receipt?

One little example: Total: $7.73

tax rate: 7.5%

I'd like to split out my entries in Quicken to track sales tax charges, but many of my charge card receipts have only the grand total, and the "other" receipt for the store is missing. Any help would be greatly appreciated by this frustrated granny!