Maintaining Orchard Ratios

A farmer operates a small orchard of apple trees and a vineyard of grapevines. The number of apple trees is represented by the variable $a$, and the number of grapevines is represented by the variable $g$. The farmer has established a ratio between the apple trees and grapevines such that for every 3 apple trees, there are 2 grapevines.

The farmer wishes to plant additional apple trees and grapevines while maintaining this ratio. If the farmer currently has 120 apple trees and wants to maintain the current ratio when adding 30 grapevines, how many additional apple trees must be planted?