A doctor gives you three pills telling you to take one every half hour.
How many minutes would the pills last?
60. Start with the 1st pill, 30 minutes later take the 2nd, then 30 minutes for the 3rd.
1st pill... 30mins later 2nd, 30 mins later 3rd, 30 mins later 3rd wears off..... therefore 90 mins, no?