The guy in the group called out. "Anyone know how much I get each week on $90,000 a year?"

Immediately I called back, "$1800 a week, less tax."

The class all swiveled around to look at me.

"How did you know that?" said another, curious. I smiled and tapped my nose.

This is how I figured out the weekly pay packet so quickly in my head.

I took the yearly amount, 90,000, and simply doubled the first 3 numbers - 900.

Which is 1800, the weekly amount.

The answer is exactly 1730, but for a quick answer in less than a second of thinking, mine is close enough.

Easy when you know how! But no use when you need to win the lottery.