The probability of the first number matching is 1/21. That leaves 20 numbers in which to pick the correct match for the second number. Therefore, the chance of selecting a match for the second number is 1/20. So the combined chance is 1/21*1/20. If we call the numbers A and B, it doesn't matter which one comes first so since there are only two ways, AB or BA of picking two numbers, we need to multiply the combined chance by 2: so we have 1/21*1/20*2=1/210 i.e., one chance on 210 (the chance of picking AB or BA).