After two half-lives or 60 years, 7.5 g of the element will be left.
<u>Explanation:</u>
<u>Half-life:</u>
In simple words, Half-life can be defined as the amount of time needed for a quantity to fall to half its value as contained at the beginning of the time period.
In this problem the half-life of the element is thirty years, then after thirty years half of the sample would have decayed and half would be left as it is.
After thirty years (The first half-life ) 30 /2 = 15 g declines and 15 g remains disappeared.
And after another sixty years (The two half-lives) 15 /2 = 7.5 g declines and 7.5 g remains disappeared.
After two half-lives or 60 years, 7.5 g of the element will be left.
