The riddle
What Can Be Wasted but Never Bought?
Everyone starts equal
A day holds 24 hours, which is 1,440 minutes or 86,400 seconds, and no one gets more. A student, a baker and a pilot all share the same 24. A week has 168 hours and an ordinary year 8,760. Money can buy a watch, a calendar and a faster train, but nothing that adds an hour to the day. Hours can be spent well or badly, which is the sense of "wasted" in the riddle. Shops sell stopwatches and hourglasses, never extra hours.
What about buying time
English does have the phrase "buy time", meaning to delay something so that more time becomes available for another purpose. It describes a delay, though, and no purchase of actual hours takes place. Merriam-Webster lists the idiom with that meaning, and the riddle can ignore it without a flaw.
A neighbor riddle
Another riddle that turns on one everyday word is what has hands but cannot clap.
Counting your own day
Try the numbers on a real schedule. Subtract eight hours of sleep from 24 and 16 waking hours remain, which is 960 minutes. Over a week of such days the waking time comes to 112 hours. Writing down where those hours go is a quick way to see what "wasted" means to one person. Save a single hour a day and the total reaches 365 hours in a year, which is more than 15 full days.