posted by Jacob .
The half-life of hafnium-156 is 0.025 s. how long will it take a 560 g sample to decay to one-fourth it's original mass?
See above to one of your other screen names.
Dr. Bob, I don't need ur criticism now!! I need your help to resolve these problems
I assure you there was no criticism. Those equations I gave you above will do them all. If you have further problems, please explain what you don't understand and we can help you through. You must understand that this is a help board and if we show you how to do one problem, then the others like that one can be done on your own. And for what it's worth, keeping the same screen name helps us help you faster and with better results.