I remember a friend of mine who is an IT teacher being roped in by the school to help cut costs of electricity. He and a Maths teacher who was also head of accounts, decided to work out how many lightbulbs they needed around the school.
They went into the main hall of the school, and the Maths teacher began counting each fixture... 1, 2, 3... Meanwhile my friend clocked that there were 8 rows of 5 fixtures, and said "40"!
The Maths bloke scowled, and said, "No, I want an accurate count, not an estimate"