Are you as smart as an 11 year old?

F

FloatLeft

Well-known member

Jun 12, 2012

1,603

• May 7, 2014

• #1

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

It's 'canard' Del

May 8, 2013

895

Burgess Hill

• May 7, 2014

• #2

A 2 b 4 c 6 d 1 e 8

 

F

FloatLeft

Well-known member

Jun 12, 2012

1,603

• May 7, 2014

• Thread starter
• #3

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

 

The Legend that IS Lawro

It's 'canard' Del

May 8, 2013

895

Burgess Hill

• May 7, 2014

• #4

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.

 

F

FloatLeft

Well-known member

Jun 12, 2012

1,603

• May 7, 2014

• Thread starter
• #5

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

Well-known member

Oct 24, 2005

50,201

Goldstone

• May 7, 2014

• #6

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

 

L

leigull

New member

Sep 26, 2010

3,810

• May 7, 2014

• #7

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

Everything in Moderation

Oct 2, 2013

10,201

The Race Hill

• May 7, 2014

• #8

Plooks said:

You can work it out by substitution.

Click to expand...

I see what you did there

 

Guinness Boy

Tofu eating wokerati

Helpful Moderator

NSC Patron

Jul 23, 2003

34,187

Up and Coming Sunny Portslade

• May 7, 2014

• #9

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

 

M

Martlet

Well-known member

Jul 15, 2003

679

• May 7, 2014

• #10

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

 

F

FloatLeft

Well-known member

Jun 12, 2012

1,603

• May 7, 2014

• Thread starter
• #11

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

 

Hamilton

Well-known member

NSC Patron

Jul 7, 2003

12,461

Brighton

• May 7, 2014

• #12

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

not my real name

Jan 29, 2009

1,884

Flitwick

• May 7, 2014

• #13

This is how I did it

 

The GIF dude

New member

Mar 22, 2013

202

Sidwell

• May 7, 2014

• #14

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

New member

May 5, 2012

1,731

• May 7, 2014

• #15

FloatLeft said:

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

Click to expand...

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

 

Last edited: May 7, 2014

SIMMO SAYS

Well-known member

Jul 31, 2012

11,716

Incommunicado

• May 7, 2014

• #16

:

smeariestbat said:

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

Click to expand...

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