Last month, Margo bought a tree that grows 2.5cm each day. It was 5cm tall when she bought it and now it is 65cm tall.

Margo bought that tree when it was 5cm,

If you are calculating how long she had had that tree than the answer is 24 days (2.5 * (65-5))

However if you are calculating what is the age of that tree then it's 26 days.

I am pretty sure this is right answer