Question

A sample of phosphorus-32 has a half life of 14.28 days. If 25g of this radioisotope remain unchanged after approximately 57 days, what was the mass of the original sample?

Answer 1

Roughly our half lifes have elapsed. In each half life half of the sample has decayed to half the mass it had before. You don't need to use equations to figure this one out. Just work backward and double the sample mass four times.