Iodine-123 has 1/2 life of 13.27hrs how many half lives for 160mg sample of iodine123 to decay to 5mg?

You can solve this type of question with an exponential function, but this one is easier than most. Look at the fraction 5/160: it's the same as 1/32. So, after the first half life, 1/2 is left, after the second, 1/4 is left, then 1/8, ... How many does it take to get down to 1/32?