1, "Baidu knows" If 0 is not used, there are 9× 8× 7× 6 = 3024 permutations.

2. If only+-×÷ is used as the operation symbol without brackets, various formulas will have many solutions.

I counted a few:

1、7*9+6 17*9-6 1=5555

2、623/7*62+37=5555

3、5+94* 1*59+4* 1=5555,5+94* 1*59+4/ 1=5555,5+94/ 1*59+4* 1=5555

5+94/ 1*59+4/ 1=5555

4、5+84+65*84+6=5555,5+84*65+84+6=5555

5、5-7*3*9+5739=5555,5739+5-7*3*9=5555