# No love for 2 and 3

Form two numbers. The sum of them must be $100$.

For the first one you must use the digits $4$, $5$ and $6$ taken exactly once in this order. You cannot use other digits.

For the second one you must use the digits $7$, $8$ and $9$ taken exactly once in this order. You cannot use other digits.

The operations you may use for forming each of the numbers are $x + y$, $x - y$, $x \times y$ and $x \div y$. Division is math division, you cannot use computer integer division where 4/5 = 0. Operator precedence is respected. Each operation may be used multiple times

You cannot use other symbols (except $456789+-\times\div)$

Example:

• 1st number: $4\times5-6$
• 2nd number: $7+89$

Sum is wrong $14+96=110 \neq 100$, but everything else is ok.

• the titles holds no clues. It's just fun (or lack of inspiration, take it as you will). It alludes to the fact that the digits $2$ and $3$ are the only ones not mentioned in the question. Sep 12, 2018 at 0:14

$45_{6} = 29$

$78_{9} = 71$

$29 + 71 = 100$

• Excellent! Probably the expected answer Sep 13, 2018 at 8:32
• Yes, that's my solution. Congratulations! Sep 13, 2018 at 10:29

Implementing lateral thinking

The question does not specify which base, $B$, we are in. Only that we are using the digits $4$, $5$, $6$, $7$, $8$ and $9$ so this means that $B \geq 10$. I will assume $B=12$ so that we additionally have the digits $a$ and $b$.

Then one answer is

$4 \times 5 \times 6 = a0$
$7+8+9 = 20$

$a0 + 20 = 100$

• not the solution I had in mind but it does respects all the requirements. As far as I am concerned it's correct and it's the best solution yet. Sep 12, 2018 at 13:01

That should work:

Number 1: $4 \times (-5 + 6) = 4$
Number 2: $7 + 89 = 96$

• Nice answer!! :D Sep 12, 2018 at 1:50
• Negation isn't in the list of allowed operations though.
– Bass
Sep 12, 2018 at 4:04
• From my understanding, negation and "(", ")" are not allowed. So this should be wrong. Sep 12, 2018 at 8:51
• Good try. However, parenthesis aren't allowed because you can't use their symbols ( ). Also, negation even if it uses the allowed symbol $-$ isn't an operation you can use. Sep 12, 2018 at 8:58
• Sorry all, I didn't read the question carefully enough. Sep 12, 2018 at 9:08

With a bit of lateral thinking:

1st number: $4 \times 5 - 6 = 14$
2nd number: $7 - 8 - 9 = -10$

$14 + (-10) = 4$

Decimal to Binary: $(4)_{10} = (100)_{2}$

• I like your answer. A step in the right direction. And your answer confirms that the lateral thinking tag is appropriate. Sep 12, 2018 at 9:05
• Wait, so this is not your expected answer? Sep 12, 2018 at 9:08
• no, it's not the answer I had in mind Sep 12, 2018 at 9:08
• Hmmm, ok. How about a small hint: Do we need multiple numeral systems for your solution? Sep 12, 2018 at 9:12

With some brute force I can confirm what we thought already: we do need some lateral thinking to solve this challenge!

In bash, try all combinations of operators (+, -, * and / and the concatenate character which is an empty string: ' '). No negation, no brackets.

Then print the ones where the outcome is between 95 and 105:

for a in + - \* / ''; do
for b in + - \* / ''; do
for c in + - \* / ''; do
for d in + - \* / ''; do
echo 4${a}5${b}6 + 7${c}8${d}9 = $(echo "scale=3 ; 4${a}5${b}6 + 7${c}8${d}9" | bc) done done done done | awk '$NF>95 && $NF<105,1'  Output: 4+5+6 + 78+9 = 102 4+5-6 + 7+89 = 99 4+5*6 + 7*8+9 = 99 4+5*6 + 78-9 = 103 4+5/6 + 7+89 = 100.833 4-5+6 + 7+89 = 101 4-5/6 + 7+89 = 99.167 4*5-6 + 78+9 = 101 4*5/6 + 7+89 = 99.333 4/5+6 + 7+89 = 102.800 4/5*6 + 7+89 = 100.800 4/5/6 + 7+89 = 96.133 4/56 + 7+89 = 96.071 45+6 + 7*8-9 = 98 45-6 + 7*8+9 = 104 45/6 + 7+89 = 103.500  So no luck here yet! • Also,$(4\times 5 + 6)+(78-9)=95$is another close one :D Sep 12, 2018 at 11:48 I doubt this is what you're looking for, and is really stretching the limits of interpreting what you said, but:  1st number: 4×5−6 2nd number: 7+89  Now, only using the allowed digits, the sum of 4 (1 is not an allowed symbol and therefore 14 becomes 4) and 96 is 100 Similar to npkllr's answer$- 4 + 5 - 6 = - 578 - 9 = 69-5 + 69 = 64DEC(64) = OCT(100)\$

• Negation isn’t allowed Sep 12, 2018 at 13:56

4*5/6 = 3.33
7+89 = 96

3.33 + 96 ≈ 100

• Welcome to puzzling.SE! Unfortunately, the result must be exactly 100 (moreover 99.333 ≈ 99, not 100). Sep 12, 2018 at 13:30
• @xhienne if ≈ stands for "approximately equal to", or "almost equal to" why is 99.333 ≈ 100 wrong? its still almost equal to the 99.333. ≈ does not mean round down. And even so, i could also go with 99 ≈ 100. Regardless of wether the sum is exactly 100.
– Ano
Sep 13, 2018 at 6:16