# Maths home work help

(17 Posts)
Choufleur Fri 19-Jun-20 11:34:19

Help! How do you work out x and y?

10x + 7y = 143

OP’s posts: |
dementedpixie Fri 19-Jun-20 11:57:24

dementedpixie Fri 19-Jun-20 12:00:46

or have you just to express it in terms of x and y fir e.g.
10x + 7y = 143
10x =143 - 7y
X = (143 - 7y)/10 (that's the top divided by 10)

10x + 7y = 143
7y = 143 - 10x
Y = (143-10x)/7 (the top divided by 7)

Panticus Fri 19-Jun-20 12:03:21

I'm not sure you can. Is there another equation that goes with it?

You can narrow it down if x and y can only be whole numbers. If that is the case, then x can only be somewhere between 1 and 13. You then need what is left (ie 143 - 10x) to be a multiple of 7.

So if you work through it, that knocks out x = 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, 13

You're left with x = 1 or x = 8. Therefore y = 19 or y = 9

I'm not sure how you can take it any further than that.

RedCatBlueCat Fri 19-Jun-20 12:03:42

What age??
Trial and error, I'd guess?

7y must end in a 3 to get the 3 in the final answer. So 9x7 is 63. Y is 9, making X 8.

dementedpixie Fri 19-Jun-20 12:05:56

what is the actual question? Are you finding the actual number or just expressing in terms of X and Y?

LizzieAnt Fri 19-Jun-20 12:12:43

I agree with @RedCatBllueCat
y must be 9 and x must then be 8. No other combination will work, at least if x and y are whole numbers. What you have to do is count up in multiples of 7 until you get a number that ends in a 3. Under 143, 7×9=63 is the only possible option. Thus y=9 and hence x=8.

Fri 19-Jun-20 12:15:48

There are infinite possible answers, as @Panticus says there are two solution for positive whole numbers. There are other solutions if you allow negative whole numbers:
(x,y)=
(1,19)
(8,10)
(15,-1)
(22,-11)
etc
ie keep adding 7 to x and subtracting 10 from y..

or you can go the other way
(1,19)
(-6,29)
(-13,39)
etc
ie subtracting 7 from x and adding 10 to y.

Normally (GCSE) there would be a second equation involving x and y, and you would solve them simultaneously for the (x,y) that fits both.

Fri 19-Jun-20 12:18:02

Whoops, one error, I meant:
(x,y)=
(1,19)
(8,9)
(15,-1)...

@LizzieAnt - you missed the other solution with x and y both being positive whole numbers, namely x = 1, y = 19.

TeenPlusTwenties Fri 19-Jun-20 12:18:12

Broadly speaking, you have given us 1 equation but with 2 unknowns.

So to solve it you must have further information or constraints (eg some posters say they are assuming x and y must be positive integers).

Otherwise there is an infinite amount of solutions that join together to make a straight line.

TeenPlusTwenties Fri 19-Jun-20 12:19:33

Fri 19-Jun-20 12:24:03

Hi, Teen. I beat you on to a maths thread for once! LizzieAnt Fri 19-Jun-20 12:25:28

Oops, so I did needsleepz Fri 19-Jun-20 13:15:22

You express one on terms of the other then substitute. Trial and error is easier though!

10x + 7y = 143
x = (143-7y) / 10

10 (143-7y / 10) + 7y = 143
Etc

Panticus Fri 19-Jun-20 13:36:16

@needsleepz you need 2 equations to be able to substitute. If you work through your example you get 143 = 143

Sat 20-Jun-20 16:40:28

@Choufleur - did any of the above help? xsquared Sat 20-Jun-20 16:49:18

If you want to solve for x and y, then you'll need another equation.

Depends what the question is as well. Is this part of a simultaneous equations question or is this a straight line graph that you need to plot points for?

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