3. Xiaoming goes to the supermarket to buy milk. If he buys fresh milk at 6 yuan per box, the money he brings is just enough; if he buys yogurt at 9 yuan per box, the money is also just enough, but he buys 6 fewer boxes than fresh milk. Xiaoming brings a total of yuan.
【Answer】108 yuan
【Key Point】Solving application problems by setting up equations
【Analysis】
Let Xiao Ming be able to buy $x$ boxes of yogurt, then he can buy $(x+6)$ boxes of fresh milk;

According to the problem, we can set up the equation: $6(x+6)=9x$, solving for $x$ gives $x=12$; so Xiao Ming brought a total of $9 \times 12=108$ yuan.