Question

Plutonium-238 has a yearly decay constant of 7.9 × 10^-3. If an original sample has a mass of 15 grams, how long will it take to decay to 12 grams?

Answer 1

So that's your formula where A is the final decay amount, A0 is the initial decay amount you have, t is time, lambda is the decay rate. Plug and chug your numbers. The only value you don't know of course is t. I'll work it out and post but you should do it as well based on what I gave you so you know it :)