Substance Z has a half-life of 10 years. You currenly have 4 grams of Z.

How much will you have 20 years from now? 25 years from now?

Ok... so after 20 years it will be 1 grams right?

but how come this equation doesn't give me the answer 1 gram?

$\displaystyle Z= (\frac{1}{2})^\frac{t}{10}$

What should be the correct eqution for it?