Microsoft Excel problem

I don't use this programme a lot and I'm trying to organise data on it for my Geography coursework. I've got around 600 numbers (they represent the median axis of rocks if anyone's interested) and I want to categorise them into these groups so I can create a histogram (the data is only in multiples of 5):

5-10mm
15-20mm
25-30mm
35-40mm
45-50mm
55-60mm
65-80mm
85-100mm
≥100mm

I don't know the formula or whatever to get excel to do this automatically to every piece of data and I've looked around the net and tried excel help and can't find anything! Please if anybody knows how to do this can you help me. In the end I hope to just be left with a list of 600 categorised results and then I can use the COUNTIF formula to find the frequencies of each category per group (there are 72 measurments per group and I'm comparing 8 groups).