After two years, you would have 400 grams of a radioisotope starting with 800 grams and a half-life of 2 years. After eight years, the amount would further reduce to 50 grams due to the sequential halving that occurs during each half-life period.
If you start with 800 grams of a radioisotope that has a half-life of 2 years, after one half-life period, which is 2 years in this case, you would have half of the initial amount. Therefore, after two years, you would have:
800 g / 2 = 400 g
After eight years, which is equivalent to four half-lives (since 8 years divided by 2 years per half-life equals 4 half-lives), you would calculate the remaining amount by dividing 800 grams by 2 four times (since each half-life halves the remaining quantity):
First half-life: 800 g / 2 = 400 g
Second half-life: 400 g / 2 = 200 g
Third half-life: 200 g / 2 = 100 g
Fourth half-life: 100 g / 2 = 50 g
So, after eight years, you would end up with 50 grams of the radioisotope.