Take two 9s to see if they are even (once).

If it is flat, look for it within 7 miles.

Divide 7 into three parts: 3, 3 and 2.

Take two 3s to see if they are balanced (quadratic).

Take 2 when it is flat, and you will know (three times)

If it is uneven, take three and two to see if it is balanced.

You only need to know once (three times)

If two nines are not even, take the light nine.

Divided into three parts: 3, 3, 3.

Take any two scales (quadratic)

Ping, just look for it in the third group and weigh it again (three times)

The light box is found uneven, which means it can be weighed once (three times).

① Take out 6 boxes of biscuits, 3 boxes on the left and 3 boxes on the right.

The first case: if the left side is heavy,

② Take out two of the three boxes on the left and put them at both ends of the balance. If the left side is heavy, the box on the left side is the right side, otherwise, if it is balanced, the remaining box is the right side.

The second case: if the right side is heavy,

Ditto.

In the third case, if we say balance,

The heavier biscuits are in the remaining two boxes.

As long as you multiply it at the left and right ends, which box weighs is which box.