I recently came across a certain question which I was not able to solve.
It was as follows:
7 × 3 = 5
9 × 5 = 41
15 × 11 = ??
Can you tell me the logic behind this?
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Sign up to join this communityI recently came across a certain question which I was not able to solve.
It was as follows:
7 × 3 = 5
9 × 5 = 41
15 × 11 = ??
Can you tell me the logic behind this?
The rule is:
Take the "normal" multiplication, and sum the squares of the digits
Explicitly:
$7 \times 3 = 21; 2^2 + 1^2 = 5,$
$9 \times 5 = 45; 4^2 + 5^2 = 41$
and so
$15 \times 11 = 165; 1^2 + 6^2 + 5^2 = 62.$
Of course, with a question like this there's no way to know for sure that this is the definitive 'right' answer, as you could come up with any number of different rules that give $7 \times 3 = 5$ and $9 \times 5 = 41$. But this "feels'' like it's the intended solution.