We know that an empty crate weighs 4.2, and the weight of each avocado is 0.3
So we can form an equation: 15.0=4.2+0.3x
(Where x is the number of avocados)
Move 4.2 to the other side as a negative
10.8=0.3x
Divide both sides by 0.3
Therefore, the amount of avocados that can be added is 36