Final answer:
After 30 minutes, there is 2.5 g of the element remaining.
Step-by-step explanation:
To calculate how much is left of the element after a certain amount of time, we can use the concept of half-life. Half-life is the time it takes for half of a sample to decay. In this case, the half-life of the element is given as 10 minutes.
Since 30 minutes have passed, that means there have been 3 half-lives (30/10 = 3). Each half-life reduces the amount of the element by half. So, after 3 half-lives, there is only 1/8 (1/2 * 1/2 * 1/2) of the original amount remaining.
Therefore, there is 20 g * 1/8 = 2.5 g of the element left.