Are you as smart as an 11 year old?

[FloatLeft]

Something to take your mind off tomorrow...

Now my son is coming to the end of primary school I find that I'm having to relearn maths to help him with his homework. This one he came back with today and I couldn't think of a scientific way of working this out other than drawing out a grid of combinations (and a bit of educated guesswork).

Is there a correct and methodical way of working this out:

Lua Lua pays £21 for five presents.

For present A & B he paid at total of £6
For B & C he paid a total of £10
For C & D he paid a total of £7
for D & E he paid a total of £9

How much did Lua Lua pay for each present?

 

[The Legend that IS Lawro]

A 2 b 4 c 6 d 1 e 8

 

[FloatLeft]

You're correct but how did you work it out?

 

[The Legend that IS Lawro]

Bit of a cheat by elimination. A had to be low (A+B less than B+C) but obviously different to B. That then leads on to the rest, if A had been 1 would have taken a few secs longer.

 

[FloatLeft]

Ah ok. I took a bit longer than you but I used the same sort of educated guesswork, I just wondered if there was an actual method to it.

 

[Triggaaar]

An extra B:
A & B (£6) + B & C (£10) + D & E (£9) = £25
B = £25 - £21 = £4
A = £6 - £4 = £2

An extra D:
A & B (£6) + C & D (£7) + D & E (£9) = £22
D = £22 - £21 = £1
E = £9 - £1 = £8

C = £21 - A+B+D+E = £6

 

[leigull]

Yeah can only do it by a bit of trial and error myself, would be interested to know if there is a more 'mathematical' approach to it.

EDIT: Just seen Triggaar's explanation above

 

[Prince Monolulu]

You can work it out by substitution.

I see what you did there

 

[Guinness Boy]

Got the same as Lawro (honest). Work backwards seeing what it CAN'T be until you get a value for a & b. After that it all just works

 

[Martlet]

There's an easier way...
A+B+C+D+E = 21

take away (A+B)[6] and (C+D)[9] and E's the only one left= 8
You therefore know that D's 1 (because D+E = 9)
C's 6 (C+D=7)
B's 4 (B+C=10)
and A has to be 2

 

[FloatLeft]

Trigaaars method left me as confused as I was before but Martlet you are obviously a genius.

 

[Hamilton]

Line 1 + Line 4 = 15 leaving 6 left. Therefore C is 6.
Once you know C is 6 it's easy.

It's a visual maths problem to me.

 

[Bedsex]

This is how I did it

 

[The GIF dude]

A + B + C + D + E = 21

B + C = 10. D + E = 9.

21 - (10+9) = 2. Therefore A = 2.

...Which means B = 4, C = 6, D = 1, and E = 8.

Edit: Sorry just realised mine is the same method as Martlet's except working out A rather than E.

 

[smeariestbat]

Trigaaars method left me as confused as I was before but Martlet you are obviously a genius.

trigger has just explained his a bit weird, and by that i mean he hasnt shown all his working. tut tut
we know A + B + C + D + E = 21
we also know a+b = 6
B+C = 10
and d + e =9
add all those together the equation is A + B + B +C + D + E = 25
so you can see from the above where he gets the 'extra B' comment from. so you can now easily see the only difference in the two equations is the 'extra b' and the total, Which is +4 to the equation with the 'extra' B. therefore B must equal 4. with that its easy to work out the rest

 

[SIMMO SAYS]

:

trigger has just explained his a bit weird.
we know A + B + C + D + E = 21
we also know a+b = 6
B+C = 10
and d + e =9
add all those together the equation is A + B + B +C + D + E = 25
so you can see from the above where he gets the 'extra B' comment from. so you can now easily see the only difference in the two equations is the total. Which is +4 to the equation with the 'extra' B. therefore B must equal 4. with that its easy to work out the rest

This will come in handy when all of our kids are working in the fifty p shop in the high street.
Or if lucky the pound shop.
Or if my son can fit a central heating system/bathroom suite on his own.
'New Maths' came in the early seventies and apart from a few did not have any relevance to most kids.
The day my Maths teacher started to try and teach me binary & logarithms my mind shut down until footie in the playground.
This lasted five years until I could escape into the real world.
Just my opinion like

Besides Lua Lua is fecking minted and should pay a lot more for his presents