by HallsofIvy » Wed Feb 17, 2021 2:27 pm
The numbers are 2, 3, 6, 7, 8. There are 2 odd numbers, 3 and 7, and 3 are even, 2, 6 and 8.
The probability the first number is even is 3/5 and then, without replacement, there are 4 numbers left, 2 of which are even. The probability that the second number is also even is 2/4= 1/2. The probability that both numbers are even, so that there are 0 odd numbers, is (3/5)(1/2)= 3/10.
The probability the first number is odd is 2/5 and then there are 4 numbers left, 3 of which are even so the probability the second number is even is 3/4. The probability the numbers are odd and even, in that order is (2/5)(3/4)= 3/10 and the number of odds is 1.
The probability the first number is even is 3/5 and then there are 4 numbers left of which 2 are odd so the probability the second number is odd is 2/4= 1/2. The probability the numbers are even and odd, in that order, is (3/5)(1/2)= 3/10 again.
The probability that the number are "even and odd" or "odd and even", so that the number of odds is 1, is 3/10+ 3/10= 6/10= 3/5.
The probability the first number is odd is 2/5 and then there are 4 numbers left, of which 1 is odd so the probability the second number is also odd is 1/4, so that the probability the number of odds is 2, is (2/5)(1/4)= 1/10.
P(0)= 3/10, P(1)= 6/10= 3/5, P(2)= 1/10.
As a check, note that the sum of these is 3/10+ 6/10+ 1/10= 10/10= 1.