Hello!
A half-life is the amount of time it takes half of an element to decay. For example, after one of carbon's half-life, half of carbon would've decayed.
Now, we can calculate how much has decayed if 5 half lives have passed.
1 -> 50% (1) -> 25% (2) -> 12.5% (3) -> 6.25% (4) -> 3.125% (5)
Therefore, there would be 3.125% left of iodine. However, your question is asking what percent has decayed. Therefore, subtract 3.125 from 100% to get your final answer.
100 - 3.125 = 96.875
Therefore, your final answer is 96.875% of iodine has decayed if 5 half lives have passed.
Hope this helps!
