There were 150 mg of a radioactive material stored at the start of the year 2000. The material has a half-life of 15 years. How much radioactive material will there be at the start of 2040?

There were 150 mg of a radioactive material stored at the start of the year 2000. The material has a half-life of 15 years. How much radioactive material will there be at the start of 2040?