A little over a year ago, I teased Grey Matters readers with a mystery skill. First, they had to learn to easily multiply by 63, then learn how to easily multiply by 72. The skill itself was revealed to be how to roughly convert any whole number of years into seconds!

In this post, you'll learn a similar skill: how to convert weeks into minutes instantly!

Back in the days before computers and calculators, this was a popular feat among entertainers who performed as human calculators. It was quick and direct to perform, yet was highly impressive to audiences.

One week has 7 days, and each day has 24 hours. Every hour, of course, has 60 minutes, so if we multiply out 1 week × 7 days/week × 24 hours/day × 60 minutes/hour, we get 10,080 minutes in a week. The number 10,080, as it happens is very easy to multiply by almost any number of weeks. If you keep the number of weeks at or below 124 (about 2.37 years), the numbers are even easier to work out.

STEP 1: Ask for any number of weeks less than 2 years (104 weeks). As an initial example, we'll say an audience member gave the number 36.

STEP 2: Write down the number they just gave you. In our example, you'd write 36.

STEP 3: Multiply this number by 8 in your head, and write this result to the immediate right of the first number you wrote. This is simpler than it sounds; all you have to do is double the number 3 times. For 36, doubling once gives you 72, doubling a second time gives you 144, and doubling a third times gives you 288. Writing 288 next to the 36 you wrote earlier gives you 36288.

NOTE: In step 3, it's very important to always treat the answer as a 3-digit number. For weeks from 13 to 104, it will be, but for weeks from 2 to 12, it will be a 2-digit number. You can change this into a 3-digit number simply by adding a 0 to the left of it. If you're given 7 weeks in step 1, you write down the 7 as in step 2, then multiply 7 × 8 to get 56, which becomes 056. You would write 056 as your step 3 answer, giving 7056, and then continue with step 4.

STEP 4: Write a zero to the immediate right of the other numbers, add commas where appropriate, and you're done! In our example, we add the zero to the right, giving us 362880. With commas, that result is 362,880. This means that there are 362,880 minutes in 36 weeks!

With a little practice, you'll be astounded as to how quickly you can pick this impressive skill up. You can quiz yourself by having Wolfram|Alpha give you a random number of weeks from 2 to 104, and then use it to verify whether you've worked out the correct answer.

HANDY BONUS TIP: You can make this more impressive for an audience by having someone with a calculator verify this in a long, drawn-out manner. Tell them to put in the number of weeks given, then multiply by 7 for the number of days in a week, then multiply by 24 hours in a day, and then multiply by 60 minutes in a week. Multiplying it out the long way makes this feat seem more difficult, as you're hiding the simple 10,080 conversion factor.

Try it out and amaze your friends!