Quantity of radioactive material after 20 years decayed to 50 grams, after 40 years it decayed to 20 grams.What was the amount of radioactive material to start with.

In problems like this usually we were finding half life or time but never initial amount.

Radioactive decay is given by

N=No e^kt

I know that I have to find N at t=0. My problem is solving for k from the above equation.

Thank you.