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Wise mathematical Mumsnetters - variable equations help

(8 Posts)
Tue 25-Sep-12 13:31:19

They've been doing variable equations and things with brackets and x and y - all totally above me. They were given 2 questions to answer and she's managed one - correct too as unknown to dd, I have the answer book (but it doesn't show the working out bit so need help where she's now stuck)

In 16 years a father will be twice as old as his son. Together, they add up to 40 years today. How old are they?
How does she work it out if x is the son's age?

professorpoopsnagle Tue 25-Sep-12 13:35:32

In 16 years the total age will be 72 (40+16+16)

At that point, F (father) is equal to 2x son. So together they equal 3.

72 years/3 equals 24. So in 16 years time the father will be 48 and the son 24.

Now minus 16 years to now, father is 32 and son 8. This adds up to 40.

professorpoopsnagle Tue 25-Sep-12 13:37:08

Just amending- second line should say

Together they equal 3x. (So 3x = 72, and 72 divided by 3 = 24)

Tue 25-Sep-12 13:43:00

professorpoopsnagle wow that was quick! How I wish I could THINK like that. Maths just hurts my brain.

Will go and see if dd understands it better now (she's sitting over the book now) and if not I'll give her your explanation! Thank you.

Tue 25-Sep-12 13:59:56

Does this make sense for the question?

x+16+2.(x.16)=40+2.16

busybee18 Tue 25-Sep-12 14:11:36

Equation 1: X+16 = 2Y
Equation 2: X+Y = 40
Where X = father’s age & Y= son’s age

Move X in equation 2 to the other side gives you : Y = 40 – X

Now replace Y in equation 1 with 40 – X

Equation 1 becomes: X+16 = 2(40-X)
X+16=80-2X
3X=96
So X=32

Now using equation 2: 32+Y=40
So Y=8

albird Tue 25-Sep-12 14:18:59

My son in year 8 too

I think he would say

sons age = x
fathers age = y
x+y = 40
so y = 40 - x

In 16 yrs time:

son age x+16
father y+16

y+16 = 2(x+16)

substitute y = 40 - x

40 - x +16 = 2x + 32 (+x both sides and bring numbers other side)

24 = 3x

x = 8

Tue 25-Sep-12 14:25:54

Thanks albird and busybee18 how I wish I had listened more in maths!

I don't get it, although I understood professorpoopsnagle's way of working it out but dd seems to have understood the principal theme of writing the equation (although she's still abit stuck) I think she's getting there.

Thanks again.

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