Approximately many half-lives have passed for each other

For element A, approximately 5 half-lives occured

Element B went through 1 half-life,

Element C sees about 2 half-lives

Element D passes 4 half-lives

Explanation:

Half lives are the periods in which an element is reduced by half of its original amount (due to radiation)

When looking at this table, simply count the number of times you see that the element has been halved. If the data doesn't show the exact value, see if it falls in between two values that were plugged in. If it does, count it!