What you need to do is find 1/8 of 50 you can just divide 50 by 8 to get 6.25 so now you have to find how many days it will take till there are 6.25 grams of iodine left every 8.1 days its mass is split in half so start splitting it in half and every time you do, you add 8.1 days 50/2 =25 8.1 25/2 =12.5 + 8.1 12.5/2= 6.25 +8.1 now you have reached 1/8 of the original amount of Iodine-131 so to find how long it took just add 8.1+8.1+8.1 (this is the same as 8.1x3) which equals 24.3 it will take 24.3 days for Iodine 131 to decay to 1/8 of its original mass.
(good luck on the regent if thats what your studying for :)
