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I could use gaussian elimination if I make some assumptions or does any one have another suggestion?

  • 4
    $\begingroup$ It looks like the symbol means you subtract the second number from the first and then subtract two $\endgroup$ Commented Nov 15, 2019 at 1:32
  • 5
    $\begingroup$ Define that symbol $\circ$ as an operator of two inputs such that $a\circ b= a-b-2$. $\endgroup$ Commented Nov 15, 2019 at 1:33
  • 1
    $\begingroup$ that's a bruh moment $\endgroup$ Commented Nov 15, 2019 at 1:36
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    $\begingroup$ Of course there are infinitely many functions satisfying $f(10,3)=5, f(8,1)=5, f(6,2)=2$ so technically $f(9,2)$ does not have a unique value. However, assuming the child hasn't learnt multiplication or division yet, you basically have $10 ± 3 ± a = 5$ and so on for the other equations, so there aren't too many possibilities. $\endgroup$
    – Toby Mak
    Commented Nov 15, 2019 at 1:39

2 Answers 2


Andrew Chin's answer definitely works. Define $x\boxdot y$ by $$x \boxdot y = x - y - 2,$$ and it fits the data given.


Say the first number is x, the second is y, and the answer is A.

I believe the formula is A = x-(2+y).

Another that works is A = x-y-2

According to this logic:

a) 9 ● 2=5

b) ● = -2-

I hope that answers your question.

  • 5
    $\begingroup$ Your two formulas are the same. $\endgroup$
    – user169852
    Commented Nov 15, 2019 at 2:31
  • $\begingroup$ I know they're the same, I was just giving two examples. $\endgroup$ Commented Nov 22, 2019 at 23:16

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