Quite often, when a number has an even number of factors, an interesting point is

Lowest factor times highest factor = number
Second lowest factor times second highest factor = number

And so on.

If the number has an odd number of factors, all the above is also true, except that the centre factor must be multiplied by itself to get the number out.

For example, 20 has factors 1, 2, 4, 5, 10 and 20.

1 x 20 = 20
2 x 10 = 20
4 x 5 = 20

Another example, 100 has factors 1, 2, 4, 5, 10, 20, 25, 50 and 100.

1 x 100 = 100
2 x 50 = 100
4 x 25 = 100
5 x 20 = 100
10 x itself = 10 x 10 = 100

Here, we do not know yet if the final factor is between 1 and 6 or is above 6. Let's first assume the missing factor is in between 1 and 6. Then, we see the following.

1 x 6 = 6
2 x 4 = 8

Of course 6 is not equal to 8, so the missing factor cannot be between 1 and 6. It has to be above 6, and the number to be found is equal to the missing factor as well.

We do not have the last factor, so we cannot multiply 1 by this factor.

However, 2 x 6 = 12
3 x 4 = 12

So the number must be 12, with the final missing factor being 12.