the half-life of isotope X is 2.0 years. How many years would it take for a 4.0mg sample of X to decay and have only 0.50 mg of it remain

After 2.0 years, the 4.0 mg sample decays to 2.0 mg.

After another 2.0 years (total 4.0 years), the 2.0 mg sample decays to 1.0 mg.

And after another 2.0 years (total 6.0 years), the 1.0 mg sample decays to 0.50 mg.

So it takes 6.0 years for 4.0 mg to decay to 0.50 mg.