# This question appeared in a mate's child's homework and it's broken my brain.

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

• It looks like the symbol means you subtract the second number from the first and then subtract two Commented Nov 15, 2019 at 1:32
• Define that symbol $\circ$ as an operator of two inputs such that $a\circ b= a-b-2$. Commented Nov 15, 2019 at 1:33
• that's a bruh moment Commented Nov 15, 2019 at 1:36
• 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. Commented Nov 15, 2019 at 1:39

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-