Greetings everyone!

I would like to ask for any online links for Nuclear Chemistry? I have been boggled by a certain Nuclear Chemistry problem. For example, you're given a certain nuclear activity of 2000 cpm for a mass of 50 mg, and 500 cpm for a mass of 60 mg. Then you are asked for the mass of the original sample? I saw from the Mortimer book the formula: No/N = (activity)0/(activity), but I can't figure out a formula of finding for the mass of the original sample. I'm trying to solve it without using a calculator...

Hope you can clear this up guys. Thanks.

Regards,

Tashkent