One day, I was going to school. I saw an old man. He said that he will pay me if I answered his question properly.

If we add 2 into 10 it will be 8, i.e. 10+2=8

If we subtract 2 from 10 it will be 12, i.e. 10-2=12

I forget that the old man was renovating his house. How is this possible?


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    $\begingroup$ Worst possible title ! $\endgroup$ – hivert Nov 23 '17 at 12:55
  • $\begingroup$ Maybe he forgot to add a minus sign (-) before the 10 and the answer? $\endgroup$ – Rick van Osta Nov 23 '17 at 12:56
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    $\begingroup$ This is a task for your maths teacher. When you get to school, tell him there's a strange man offering money to school children and let him deal with the problem. $\endgroup$ – Vince O'Sullivan Nov 23 '17 at 13:41
  • $\begingroup$ @VinceO'Sullivan Aside from the horrendous title, this suddenly took a very dark turn $\endgroup$ – MechMK1 Nov 23 '17 at 16:43

By renovating his house he could be painting ?

He have a wall to paint. 10m2 are already painted. By painting 2m2of the wall 8m2 left to paint.
10+2 = 8
By removing 2m2 of paint it left 12 m2 to paint. 10 -2 =12

  • $\begingroup$ Isn't that subtraction? 10m² wall - 2m² painted = 8m² yet to paint. $\endgroup$ – Phylyp Nov 24 '17 at 2:00
  • $\begingroup$ @Phylyp you add up 2m to the 10m so it make 12m painted and 8m left to paint so 10m+2m=12m painted and 10m+2m =8m left to paint $\endgroup$ – guillaume Nov 25 '17 at 4:36
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    $\begingroup$ That is a feat of logic that should go down in the annals of history. What does the first fact: 10 sq/m are already painted have to do with the second fact: If 2 sq/m are then painted 8 sq/m will be left to be painted? Are you repainting some of the wall? $\endgroup$ – Dr t Nov 26 '17 at 20:37

I think:

This can be seen as addition of electrons to a mix of
protons/electrons (Net charge $10^+$)

If we add 2 electrons to this body net charge will become $8^+$

If we subtract 2 electrons from this body and net charge will become

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    $\begingroup$ The old man don't know what are the electrons and protons. He will be confused with this answer $\endgroup$ – rudra Nov 23 '17 at 14:35

It can be years Before Christ.
10 BC + 2 = 8 BC
10 BC - 2 = 12 BC

  • $\begingroup$ If you add 10bc+2bc=12bc $\endgroup$ – rudra Nov 23 '17 at 14:37
  • $\begingroup$ @rudra he is talking about the timeline. $\endgroup$ – prog_SAHIL Nov 23 '17 at 14:55
  • $\begingroup$ @prog_sahil read the question again i have given a hint over there. $\endgroup$ – rudra Nov 23 '17 at 15:13
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    $\begingroup$ @rudra: Arguably, this fits with the tip, since Jesus was a carpenter :) $\endgroup$ – Flater Nov 23 '17 at 15:16
  • $\begingroup$ @rudra how do you know what he knows? you just met him on the road. Little you know, he can be a MIT scholar. $\endgroup$ – prog_SAHIL Nov 23 '17 at 17:46

renovating his house could mean that the old man was talking about:

Subterranean floors (underground floors): Floors below the ground level.


10 + 2 = 8 => go up 2 floors from the 10th subterranean floor and you're at the 8th subterranean floor.
10 - 2 = 12 => go down 2 floors from the 10th subterranean floor and you're at the 12th subterranean floor


Since the tag is mathematics and is a calculation-puzzle, here is a partial one

In logical binary XOR operation (which is a + inside a circle), the following holds good: 2 + 10 = 0010 + 1010 = 1000 (which is 8)

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    $\begingroup$ The old man doesn't know about computer $\endgroup$ – rudra Nov 23 '17 at 15:54
  • $\begingroup$ @rudra That's mathematics. Heavily used in computers, but mathematics nonetheless! $\endgroup$ – Olivier Grégoire Nov 23 '17 at 16:23

This question has already an accepted answer, but I would add a more generic one:

Basically anything where the 10 and the 2 have different signs works


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