Maths problem to solve while sitting around the campfire?

[BigWave]

Sorry Booney, was going to let others figure out why.
PS: It helps to have a PhD for these mind puzzles. Pity I don't

 

[BigWave]

OK, here's an arithmetic one. What's the missing number?

 

[Jaros]

15

 

[BigWave]

Sorry Jaros. No cigar (let alone Cuban).
Anyone else?

 

[Jaros]

TSOK don't like cigars...

 

[RM Outback]

15

Great minds think alike :Y: came up with the same number and don't smoke cigars

 

[BigWave]

Sorry Rob, it's not 15.

 

[BigWave]

A heads-up.
The 7 at the bottom is not a typo.

 

[Booney]

Think its 12 . Could be wrong ?
At first thought 15 and subtraction but the 7 didnt make sense . Knowing 15 was wrong and 7 was right , using the arrows worked backwards from the 7 and there it was . Anyone else see it ?

 

[BigWave]

Congrats Booney - it's 12
It's a sneaky puzzle because subtraction works all the way to the bottom line, then fails.
I've not seen anyone get it right on the first go.
Back to you..........

 

[Booney]

Happy to pass to anyone else who can see the solution . My brain hurts :lol:

 

[Manpa]

Can you please explain how it is 12, Im struggling to make it work, I must be missing something. (Im missing more than I get lately)

 

[Jaros]

Me too plse.

 

[Nightjar]

Booney.
Please help, looks like I'm not the only one who can't get their head around this one?
The 7 isn't a typo??? That must add to the mystery?

 

[RM Outback]

Congrats Booney - it's 12
It's a sneaky puzzle because subtraction works all the way to the bottom line, then fails.
I've not seen anyone get it right on the first go.
Back to you..........

Thanks for the explanation but it doesn't help me and a few others by the looks of it. Can you please provide the formula I've wasted too much time on this this morning :argh:

 

[Booney]

Ok look at the bottom line
13 with an arrow pointing to 7
and 21 with an arrow pointing to 7
So somehow these numbers need to end up equaling 7
So only way is
1 + 3 = 4. 2+1= 3

4+3=7
Now do all the other lines and you will see how I got to 12
So. 21. 36

?

2+1=3. 3+6=9

3+9=12
The thing with these puzzles is there is always a simple trick not always plain to see but once seen , seams simple

 

[Nightjar]

Now that made it easy, brain at rest. :argh:

 

[Manpa]

Thankyou.

 

[RM Outback]

Onya Booney :beer: had you not explained the formula I would have taken a bloody long time to get there :lol:

 

[Booney]

No worries guys , someone else have a go while I bask in the glory dosnt happen often :lol: