What is the value of n? ​

5?

Step-by-step explanation:

The lcm is never less than the larger number.  An easy way to generate the
pairs you're looking for is to make the larger one a multiple of the smaller
one.  Then the larger one is always their lowest common multiple.

2  and  20 . . . lcm is  20 .

3  and  39 . . . lcm is  39 .

5  and  45 . . . lcm is  45 .