Respuesta :
If it was 120 stamps in October but by the following March it was 138 stamps. Her collection grew 18 stamps between 5-6 months. If it is 6 months then that is atleast 2 stamps a month.
i know this is really late but this is for anyone else that wants to know the answer
138 -120 = 18 18/120 X 100 = 15%