how to solve simple half life? ( gr9 mathematics)?

Hey there,

Im studying for math exams (gr9) and I have a half-life question that wasnt taught to me. I did some reading on it and this is what i have so far

Amount left=Amount original(1/2) * time/time half life.

This is my problem : In 30h, a sample solution of plutonium-243 decays to 1/64 of its original amount. Determine the half life of each isottope.

Thanks in advance.

PS:and how would i simply finf the Amount left with all the given required values.