Two different numbers are selected simultaneously and at random from the set {1,2,3,4,5,6,7,8,9,10}. What is the probability that the positive difference between the two numbers is 3 or greater? Express your answer as a
common fraction.
There are:
[10 + 2 - 1] C 2 ==55 combinations as follows:
(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 8, 8, 9) >>Total = 55 combinations
The probability is: 28 / 55
The above is with replacement. This is without replacement:
There are:
[10 C 2 ==45 combinations as follows:
( 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 8, 8, 9) >>Total = 45 combinations
The probability is: 28 / 45
