Part A.
We're only told that the product of the mystery factor and the number shown is even. This is true when the number shown is odd as well as even, hence the mystery factor is even.
There are no other conditions on the product that might limit the value of the mystery factor (non-zero, or < 10, for example).
Hence, the mystery factor could be any single-digit even number. In our base-10 number system, there are 5 of them: {0, 2, 4, 6, 8}.
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Part B.
There are only 5 even single-digit numbers in our base-10 number system, so by choosing 5, I know I have all of them.