7. (7 points) Dad gives Xiaojun the same amount of pocket money every day. Xiaojun originally had some money. If he spends 10 yuan every day, it can last for 6 days; if he spends 15 yuan every day, it can last for 3 days. Xiaojun originally had $\qquad$ yuan.
【Solution】Solution: According to the problem, let the amount of pocket money Dad gives Xiaojun every day be $x$ yuan. Since Xiaojun spends 10 yuan every day and can use it for 6 days, the original amount of money Xiaojun had is $6 \times 10-6 x$. Since he spends 15 yuan every day and can use it for 3 days, the original amount of money Xiaojun had is $15 \times 3-3 x$. Therefore, $6 \times 10-6 x=15 \times 3-3 x$, solving this gives $x=5$ yuan,
$\therefore$ Xiaojun originally had $6 \times 10-6 \times 5=30$ yuan, so the answer is 30.