## Archive for December 9, 2014

### Mental Math and the MCWC How long would it take you to find the sum of 2 two-digit numbers?

Or 4 four-digit numbers?

Okay, let’s get really crazy… how long would it take you to find the sum of 10 ten-digit numbers?

You can decide whether you’ll do the calculation in your head, on a calculator, or with paper and pencil. Your choice.

With a calculator, it took me 91 seconds to find the sum of 10 ten-digit numbers.

Without a calculator, the winner of the Mental Calculation World Cup 2014 needed only 242 seconds to complete 10 problems in which participants were asked to add 10 ten-digit numbers. On average, that’s just 24 seconds to do in his head what took me a minute-and-a-half with technology.

Holy smokes!

Competitors at the MCWC do a number of mental calculation tasks. The following exercises will give you an idea of the computations that they do.

Exercise 1. Sum of 10 ten-digit numbers. $\begin{array}{r} \mathbf{4190187220}\\ \mathbf{3967093178}\\ \mathbf{8567125486}\\ \mathbf{1005683165}\\ \mathbf{3635944647}\\ \mathbf{7645865467}\\ \mathbf{3506970235}\\ \mathbf{6710259450}\\ \mathbf{2 347 894 647}\\ \mathbf{\underline{ + 4 995 420 559}}\\ \end{array}$

Exercise 2. Multiplication of 2 eight-digit numbers. $\begin{array}{r} \mathbf{18467941}\\ \mathbf{\underline{ \times 73465135}} \end{array}$

Exercise 3. Square root of a six-digit number. $\mathbf{\sqrt{530179}}$

Exercise 4. Day of the week for a calendar date. $\textbf{8 December 1721}$

Exercise 5. Multiplication of 3 three-digit numbers. $\mathbf{184 \times 734 \times 635}$

These sample exercises are taken from the examples in the MCWC 2014 Official Rules.

If you were able to complete all five of those exercises in, say, less than 3 minutes, then you might be ready for MCWC 2016. (Note that for Exercise 3, you needed to be accurate to within 5 × 10-6.)

But if you’re like me, you’ll probably want to skip the competition and keep your calculator close at hand.