To find which tree is closer to being fully grown, you must find how many centimeters remains until each tree is fully grown.
Emma's tree is currently 120 cm tall and when it is fully grown it will be 270 cm.
270 cm - 120 cm = 150 cm
This difference shows that the tree has 150 cm until it will be fully grown.
Harry's tree is 150 cm and when it is fully grown it will be 300 cm.
300 cm - 150 cm = 150 cm
The difference shows that the tree has 150 cm until it is fully grown.
Both trees have the same amount left to grow until they are fully grown. The only other factor that can really determine which tree is closer to be fully grown is if one tree is growing faster than the other. However, from the information given in the problem, that cannot be determined.