John’s heart beat at a rate of 65 beats a minute when he is awake and 50 beats a minute when he sleeps (8 hours). how many times does his heartbeat in a 24-hour period?

His heart beats 86,400 times within a 24 hour period.
60 minutes in 1 hour, so you would multiply 8 hours by 60 mins, giving you 480 mins. 24 hours- 8 (time he's asleep) will give you 16. 16 * 60 will give you 960 minutes. 480 minutes (when he's asleep) * 50 beats a minute = 24,000. 
960 (when he's awake) * 65 beats a minute = 62,400. add 24,000 and 62,400 = 82,400 beats in 24 hours.