Payment should be made directly to 7 yuan, 6 yuan, and no change is allowed.

If you can get change, 6 yuan 7 yuan is actually enough.

Extreme idea: buy things with X yuan, buy them with X 7 yuan, and get X 6 yuan back. (I believe you can)

And if you can't change it, there are the following solutions:

( 1) 1、2、3、4、5、 8、9、 10、 1 1、 15、 16、 17、 22、23、 29.

*** 15 cases cannot be realized.

Prove that when the price >; 29: 00 must be paid in coins from 6 yuan and 7 yuan.

30=6+6+6+6+6

3 1=6+6+6+6+7

32=6+6+6+7+7

33=6+6+7+7+7

34=6+7+7+7+7

35=7+7+7+7+7

And > 35 yuan, as long as you add a 6 yuan number before the payment is made in 30~35 yuan.

For example, 40=34+6 and 75=33+6*7.

So when the price >; When you are in 29 yuan, you must be able to pay with coins from 6 yuan and 7 yuan.

(2) The most expensive 29 yuan.