Answer
The fourth factor is 5
Explanation
It is given that there are only four factors. Three of them are 1, 2, 10
Here 10 is a factor, so we know that , if 10 is a factor of a number,
the we can write, 10 = 5 x 2, it implies that 5 and 2 are the factors of that number. Here 2 is given. Therefore the fourth factor is 5
The four factors are 1,2,5 and 10