Number of all cases:

6! = 720

The number of cases where only two students have the same student number and seat number:

C(2，6)*3*2* 1* 1= 90

So the probability is:

90/720= 1/8 (or 12.5%)

Ps: change the last number:

C(2，6)* 3 * 3 * 1 * 1 = 135

C (2,6) needs no explanation. If you sit in the right place, you might as well make it 5 or 6.

So 1, 2, 3, 4,

1: 3 option, you might as well set it in the second position, which leaves 1, 3, 4 positions.

Then for the option of 2: 3, we might as well set it to position 3, leaving position 1 and position 4.

Then the 4: 1 option can only enter the 1 position.

The last 3: 1 option, enter position 4.

So the probability: 135/720= 3/ 16.