Answer:
x = 
Step-by-step explanation:
Given
+
= 1
Multiply through by ab to clear the fractions
bx + ax = ab ← factor out x from each term on the left side
x(b + a) = ab ← divide both sides by (b + a)
x = 
There are 72 green pens.
There is 9 times as many white pens from 2 to 18, so there has to be 9 times as many green pens because of the ratio. 9 times 8 is 72.