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One last simultaneous equation watch

1. i have to work out :

the sum of Ann and freds ages is 28. in 7 seven years time ann will be twice as old as fred. how old are they now?
2. a + f = 28

In 7 years time Ann will be a + 7
How old would Fred be
3. So the equations are

4. yes ive worked out A+7=2f but what are their ages?
5. a + 7 is not = to 2f
6. (Original post by mmmpie)
So the equations are

Shouldn't the second equation be a+7=2(f+7)?
7. (Original post by Gemini92)
Shouldn't the second equation be a+7=2(f+7)?
Whoops. Yes

Best edit to avoid confusion...
8. please can some one answer this GCSE level question i need the equations and their ages
thank you
9. the equations have been given
10. If we just solve it for you, that doesn't help you learn how to do it.

First construct the equations, which are

Then I would simply the second one my multiplying out the bracket.

Spoiler:
Show

So the second equation becomes

then simplifying a bit more

Then you can use substitution to eliminate one of the variables. Do you know how to do that?
11. bump

can someone explain substitution to get the answer for this question
thanks.
12. (Original post by Jed123)
bump

can someone explain substitution to get the answer for this question
thanks.
The idea behind substitution is that if you have two equations in two variables, you're able to rearrange one such that it gives you a valid expression for one of your variables. You can then use this to rewrite your other equation in terms of a single variable, and thusly solve it.

For example, (I'm not going to do it for you, but here's a similar question...that you could probably find if you looked in your textbook)
Spoiler:
Show

Let's take

From the first equation, we can rearrange to get

We can then rewrite the second equation:

Simplifying the above we get to from which you should be able to solve for x giving

Then, we know that x + y = 1, so substituting in x, we get that y is also 1/2.

Can you answer your question now?

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