Final answer:
Iodine-131 has a half-life of 8 days. If a sample starts with 60 grams and there are 15 grams remaining, 16 days have passed.
Step-by-step explanation:
In radioactive decay, the half-life is the time it takes for half of a radioactive substance to decay. In this case, iodine-131 has a half-life of 8 days. We can use the formula for exponential decay to calculate the number of days that have passed.
Starting with 60 grams and 15 grams remaining, we can set up the equation:
60g * (1/2)(t/8) = 15g
Simplifying the equation gives:
1/2(t/8) = 15/60
1/2(t/8) = 1/4
Taking the logarithm of both sides, we get:
t/8 = log2(1/4)
By solving for t, we find that t = 8 * log2(1/4) = 16 days.