How long will it take for a 12.0-g sample of iodine-131 to decay to leave a total of 1.50 g of the isotope The half-life of iodine-131 is 8.07 days?

half life is 8.1 days, so it takes 8.1 days for half the iodine sample to decay. It takes another 8.1 days for half of the remaining sample (ie. 1/4th of the original sample) to decay. So it takes 16.2 days for 3/4th of the sample to decay.