7. (3 points) Xiaopang's mother went to buy apples, wanting to buy 5 kilograms. When paying, she found she was short of 3 yuan and 50 cents, so she ended up buying 4 kilograms, and had 1 yuan and 50 cents left. Xiaopang's mother brought a total of $\qquad$ yuan.
【Solution】Solution: Unit price: $(3.5+1.5) \div(5-4)$,
$$
\begin{array}{l}
=5 \div 1, \\
=5 \text { (yuan); }
\end{array}
$$

Total amount: $5 \times 4+1.5=21.5$ (yuan);
Answer: Xiao Pao's mother brought a total of 21.5 yuan.
Therefore, the answer is: 21.5.