Ava planted 9 apple trees in her backyard. That's 3 more than Lily planted in her backyard. How many apple trees did they plant in their backyards collectively?
Lily planted 9-3 = <<9-3=6>>6 apple trees in her backyard.
They planted 9+6 = <<9+6=15>>15 apple trees in their backyards collectively.
#### 15