Well, to understand what "n" is, we have to understand the relation between 18 and 27.
If we divide by 9, we'll notice that 18/27 simplifies to 2/3.
So, let's apply this to 10/n
10 over n must be the same ratio of 2/3.
10/2 is 5.
5*3 = 15.
n = 15.
Alternatively, cross multiply and solve.
10/n = 18/27
n will multiply with 18, 10 with 27.
18n = 270
/18 /18
n = 15