The number of possible combinations is an illustration of principle of counting
There are 15 possible pairs of possible combinations
From the question, we understand that the first digit is 6, 8 or 9.
This means that the new table would not have entries that begin with 5 or 7.
The table is then represented as:
6 8 9
5 65 85 95
6 66 86 96
7 67 87 97
8 68 88 98
9 69 89 99
Count the number of entries
Entries = 15
Hence, there are 15 possible pairs of possible combinations
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