# Thread: Linear Equation In One Variable

1. ## Linear Equation In One Variable

The sum of the present ages of father & son is 99 years. When the father was as old as his son is now, his age was four times the age of his son at that time. How do I find the present age of father & son?

Thanks,

Ron

2. Son is 19.8 years old; father is 79.2 years old.

Call the age of the father F, and the son's age S.

You know the combined age of the father and son:

$\displaystyle F + S = 99$ (equation 1)

And you know that the father is four times older than the son:

$\displaystyle F = 4S$ (equation 2)

So you've just got simultaneous equations.

Rearrange and substitute and you arrive at two more equations, one of which tells you the age of the father, and the other tells you the age of the son:

$\displaystyle S = 99 / 5$

...and...

$\displaystyle F = 396 / 5$

3. Originally Posted by rn5a
The sum of the present ages of father & son is 99 years. When the father was as old as his son is now, his age was four times the age of his son at that time. How do I find the present age of father & son?

Thanks,

Ron
1. Age of father today: x
Age of son today: 99 - x

2. Time difference between the two ages: x - (99 - x) = 2x - 99

3. When the father was 2x-99 years younger he had the age of his son today:
99 - x
Age of the son in the past: 99 - x - (2x-99) = 198 - 3x

4. This age of the son was a quarter of the age of his father:

$\displaystyle \boxed{99 - x = 4(198 - 3x)}$

5. Solve for x.

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# sum of the present age of a father and his son is 99

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