Sorry, this incorrect. Every 10 years the amount remaining is cut in half. At year 0 you have 20 grams, and ten years later at year 10 you have half of that, or 10 grams. At year 20 you have half of what you had at year 10, or 5 grams. Continue on in this manner, cutting the amount in half for each 10 year period, and what do you get at year 50?
To make an equation for this you can raise 1/2 to the number of half life periods to determine the fraction of material remaining. Since the half life is 10 years, and you want to know how much is remaining after 50 years, that's 5 half-life periods. So (1/2)^5 = 1/32, and hence 1/32nd of the original amount is remaining after 50 years. What's 1/32nd of 20 grams?