BrainDen.com - Brain Teasers

## Question

Saw this one on a test. Think outside the box.

9*2=13

5*3=11

4*9=21

9*4=17

What does 7*8 equal?

note: the * does not indicate multiplication. It indicates some sort of action or actions.

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• 0

double the smaller number, add to the larger

7*8 = (2x7) + 8 = 22

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double the smaller number, add to the larger

7*8 = (2x7) + 8 = 22

That won't work, 'cause 4*9 would be 17 not 21.

A variation to that theme which would work is: if second number is smaller, then [first number] + twice [second number], else, [first number] + twice second number] - 1. That is quite inelegant, and does not solve for instance of both numbers the same. Back to thinking further outside cube.

Edited by Caliban27

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I don't have a proper solution as yet but:

9*2=13 - A + 2B = C

5*3=11 - A + 2B = C

4*9=21 ???

9*4=17 - A + 2B = C

Edited by Emil

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Translate "A * B = C" into

2 * ( B + floor( (A + 1) / 2 ) ) - 1 = C

For A=7, B=8, this works out to 23

floor(x + 1) is more easily written as ceil(x)

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oops, sorry... screwed up on 4*9. it should equal 22.

REVISED QUESTION:

9*2=13

5*3=11

4*9=22

9*4=17

What does 7*8 equal?

note: the * does not indicate multiplication. It indicates some sort of action or actions.

With this in mind, Emil is very close to the answer but not quite.

• 0

7*8=23?

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