Please Help! I'm not sure what category this comes under!

Looking at the other threads, I'm pretty sure this'll seem easy to you guys. But i'm really stuck. i mean, i don't even know how to go about working this thing out.

"Katie has a new job as a primary school teacher. as part of a science experiment, she helped all 30 of her students to plant a seed each. after one week, the class measured their plants' heights to the nearest centimetre Several of the classes' plants had grown to 5cm. A smaller number had grown to 2cm. The rest, including micky's plant, had only grown 1cm. The class added up the total height of all plants.

after another week they measured again. all the plants except Micky's had doubled in size, where his was still only 1cm. The class added up the total heights of their plants which came to 131cm.

How many plants of each height were there after the first week?"

Thanks for any help in advance. :]