# Simple Arithmetic Puzzle 4. Or is it?

Can you find the value of the question mark?

• It seems to be -8100 from the last but 2nd and 3rd statements... Jul 9, 2018 at 16:59
• Does 3.4 x 1.2 also equal 141? Jul 10, 2018 at 14:17
• @JeroendeK Yes. Jul 10, 2018 at 14:28

The rule for the multiplication is

$a \times b \equiv [ 100 \ln(ab) ] = [ 100 (\ln a + \ln b) ]$
where the brackets $[ ]$ denote the Nearest Integer Function.

Examples

$56 \times 0.7 = [ 100 \ln(39.2) ] = [ 366.86\ldots ] = 367$
$12 \times 34 = [100 \ln(408)] = [601.126\ldots] = 601$
$\sqrt{2} \times \sqrt{3} = [100 \ln(\sqrt{6})] = [89.587\ldots] = 90$

How I found this solution

The $5$th and $6$th equations give the hint that this may have something to do with taking logs. Generally, if I see a function where the reciprocal argument yields the negative solution that is what I would try. Also, the number on the right seems to increase as the product of the numbers on the left increases. After messing around with logs it didn't take long to find a multiplicative factor that worked.

• (+1) Spot on again! Might want to add what the value of $?$ actually is :) Jul 12, 2018 at 9:41

I noticed something. Is it worth mentioning? I don't know. But here it is anyway:

If you actually carry out the multiplication, multiply by $10$, and then subtract by/from the answer that has been put up, you will get a number near to a square number (subtract by if larger; subtract from if smaller).

Supporting Examples:

\begin{align}56\times 0.7 &= 39.2 \tag*{\big(also equals 0.7\times 56\big)}\\ 39.2\times 10 &= 392 \\ 392-367 &= 25 = 5^2\end{align} \begin{align}12\times 34 &= 408 \\ 408\times 10 &= 4080 \\ 4080-601 &= 3479=59^2-2\end{align} \begin{align}1.2\times 3.4 &= 4.08 \\ 4.08\times 10 &= 40.8 \\ 141-40.8 &= 100.2 = 10^2+0.2\end{align}

And then you got the radicals:

\begin{align}\sqrt 2\times \sqrt 3&\simeq 2.4494897427831781\; (1) \\ (1)\times 10 &= 24.494897427831781\; (2) \\ 90-(2) &= 65.505102572168219=8^2+1.505102572168219\end{align}

Also,

In the puzzle, it appears that if $a=b$ then $1\div a=-b$. Let $a=\sqrt2\times \sqrt3$ and $b=90$.

So, my best bet from this observation is that:

$$1\times 1 = 9$$

Since

\begin{align}1\times 1 &= 1 \\ 1\times 10 &= 10 \\ 10-9 &= 1 = 1^2\end{align} And I will assume that since $1\times 1 = 1$, then in the puzzle, $1\times 1\neq 1$. We also have that $10\times 9 - 9=9^2$ so that is related.

• Not quite, although very good try! Unfortunately the answer is not $9$. It's something much more simpler, with a general formula which all of the examples given obey :) Jul 10, 2018 at 12:19
• @TheSimpliFire ah dang. Great puzzle, though! Wish I could upvote, but I have reached my daily voting limit :\ Jul 10, 2018 at 12:21