As an estimate, it’s less than 15 minutes each day. This can be computed without any calculator as follows:

There are 24 hours each day. ’24’ is near ‘25′ and 25 times 4 equal 100. So as an approximation, 1 hour or 60 minutes consist of 4% of each day. Then, 1/4 of an hour or 15-minute is 1%.

Now, 1/96 (15 minutes) is bigger than 1/100, so 15 minutes is longer than the actual 1% of a day. To check the number, we use a calculator, which says 1% of day consists of 14.4 minutes.

This method of thinking is inspired by Herb Simon, whom I had the opportunity to listen to when I was at Carnegie-Mellon. Dr. Simon’s point was that using our mind and the available information, we can figure things out quickly, without turning on a calculator or a computer. One example he told was on how to quickly figure out the circumference of the earth:

From Los Angeles to New York is approximately 3,000 miles and crossing the continental U.S. spans 3 times zones, so each time zone is approximately 1,000 miles. There are 24 times zones in going around the earth, so an estimate for the circumference of the earth is 1,000 times 24 or 24,000 miles. Googling “earth circumference” comes up with 24,902 miles. So we obtained a good estimate just using our mind and some common knowledge.

Copyright (c) 2008 by Waiming Mok