(1)∵ In the serial number, there are multiples of 20: 20,40, and those divisible by 20 are: 1, 2,4,5, 10 (in order not to count repeatedly, only 20 are counted once).

∴: Seven numbers are multiples of 20 or divisible by 20.

Then the probability that the serial number on the card is a multiple of 20 or divisible by 20 is:

7 /50

(2) unfair,

∵K can be divisible by 1 no matter what value it takes, so the probability of students with serial number 1 being drawn is 1, that is, 100%.

Obviously, the probability of students drawing other serial numbers is not 100%.

∴ unfair;

(3) Draw one first, write down the number, and then put it back. If the next number is the same as the last one, don't count it, put it back and draw it again. Repeat until 10 different numbers are drawn.

(in order to ensure that the probability of each number being drawn every time is

1/50

)

I wish you progress in your study.