Start by noticing that algebraically, your statement translates to :
(Do you see why?)
We'll solve the left side first:
(Since n is positive, 19+n is also positive, and we don't need to worry about direction of the inequality.)
On the right side:
So . Since you said n is a positive integer, we'll adjust to , of which there are 49 of them (0 and 50 aren't included).