Count the possible combinations of 2 numbers chosen from the list 99 98 97 96 95 94 93 92 91

The number of ways to choose r objects out of n objects is:
C(n,r) = (n!)/(r! * (n-r)!)

The number of ways to choose 2 numbers out of a list of 9 numbers is:
C(9,2) = 9! / (2! * 7!)
= 9*8 / 2!
= 9*8/2
= 36

Here are all the possible combinations:
98 97
98 91
98 93
98 99
98 96
98 94
98 92
98 95
97 91
97 93
97 99
97 96
97 94
97 92
97 95
91 93
91 99
91 96
91 94
91 92
91 95
93 99
93 96
93 94
93 92
93 95
99 96
99 94
99 92
99 95
96 94
96 92
96 95
94 92
94 95
92 95

It would be 15 m how i got the answer: (20x20) + b^2 = (25x25)400 + b^2 = 625625-400=225square that and get 15hope this : )

40%=2/550=1/230=3/1056=14/25