Fresh Snack is a retailer of healthy plant$-$based snacks. In $2021,$ the company added $10$ new stores to the $100$ stores it had at the beginning of the year. The company's total sales were unchanged between $2020$ and $2021.$ The prices of its products rose $7\mathrm{.}5\%$ during $2021$ but there were $3\mathrm{.}5\%$ fewer customers in that year. Based on these facts, what was the percentage change in real dollar, purchases per customer visit? The correct answer is $-12\mathrm{.}4\%,$ please explain the steps fully.