Maths homework help ..... please

[indignatio]

Ds has some maths homework - he hasn't tried to do it yet. I can work out the answers, but am doing it in such a complicated manner that he would not grasp the concepts if I tried to explain my particular way of doing it.

Please could someone explain an easy way of working out the following problem.

A goldfish costs £1.80
An angel fish costs £1.40

You need to spend £20.00 exactly. How many of each fish do you buy.

Many thanks

Shameless bump

The way I automatically went to work it out was to work out how much multiples of fish would cost e.g.

1 goldfish = £1.80
2 = £3.60
3 = £5.40
4 = £7.20

Carry on then do same with the angel fish.

Then ask your DS to spot 2 amounts, 1 in goldfish column and one in angel fish column that equal £20.

The only one I can spot is one goldfish £1.80 plus 13 angel fish £18.20 equals £20.

Hope this is useful - there's prob some complex formula type way, but at least like this your DS can see what he's doing.

2 x goldfish = £3.60
1 x angelfish = £1.40
so 2goldfish and 1 angelfish cost £5.00, so multiply by 4 to make 8 goldfish and 4 angelfish.

Or those too!
You can tell I'm still on maternity leave!

Thanks nooonit and tortoiseShell.

TS how would you explain how to find the £5 (or how you would know to use £5) to make life easier, other than by a trial and error approach.

noonit's way looks pretty sensible - the other way you could go about it is trying to make up whole pounds like tortoiseshell by saying

5 goldfish = £9

or

5 Angel fish = £7

or

2 goldfish leave a gap of 40p so add 1 angel fish = £5

or 3 angelfish and 1 goldfish = £6

actually that's a faff

Go with noonit's way!

There's no complex formula as there are lots of answers (although they may not all give whole numbers of fish!)

The way I spotted it was that 1.80 and 1.80 adds up to something ending in 60, 60+40 = 100, therefore whole number of pounds, and then spotted that it was a factor of 20. But I think with that few fish a trial and error approach is ok.

i would do as noonit suggests and have 2 columns. Keep adding the price onto the fishs as you go down so he can see how its working and then work through it by adding up both columns until you find the pair that matches £ 20.00

Not very good at explaining so hope that makes sense

Thank you all

I think I will start (if he asks for any help !) with nooonit's two columns and see if he picks up TS's £5 multiple or whether he works all the way through

Thanks again

cant you just tell them the pet shop was closed so you couldnt buy any . LOL

This is an exercise in spotting "pairs to 10", IMHO - ask DS if that is a term he recognises. That should lead him (with a bit of help) to something close to either Noonit or Tortoise's approach.

ssq you may be right

paros - dh likes your answer best - he has however written an excel spreadsheet to see which pairs make £20. HOW DO YOU HAVE A MINUS FISH - honestly men !

He has read this over my shoulder and was at pains to explain that he had discounted non integer parts of fish - still couldn't come up with real alternative answers to my 13:1 and 8:4

indignatio - that's exactly what my DH did when asked about this type of problem. I asked him why the negative entries & he said "I put those in for the sake of completeness"
Is your DH a statistician?

So presumably these answers including negative fish would involve me taking goldfish back to swap them for angel fish, or vice versa?

RB - no an Engineer (Geek variety)

DS however took a different approach and asked for money. He therefore had £20 in £1 notes (monopoly money) and exchanged notes for monopoly houses worth 20p, 40p and 80p. He then proceeded to work out the 8:4 answer by making up a pile of £1.80 fish and a pile of £1.40 fish and checking he had no money left over.

Just to pop in and join in - especially on the dhs positing negative fish! Mine did that too!!

dh & ds got stuck into something before I sent ds to bed which involved a sort of bar graph: multiples of angelfish on one axis, multiples of goldfish on the other, to produce a line where the multiples meet to hit exactly 20.00 (or, obviously, fall short of or overrun). That gave ds his second answer of 13:1 (he'd already got the 8:4).

DH is objecting to being (called DHed) a geek and has said that us women are silly - apparently of course you can have negative numbers of fish - or non integer numbers of fish - he is now shouting over my shoulder about imaginary fish - I shall now take him to bed to shut him up.

I will grant him negative fish (on the huge assumption that the fish shop will accept returns) and non-integer numbers of fish (although the mind boggles slightly as to why you would be buying those from a pet shop rather than a fishmonger) but I draw the line at imaginary fish.

Your DGeek DH needs to grasp the difference between pure and applied maths, methinks.