How to make an equation out of this problem?

Question

Right now Kim is 15 and her father is 39. In how many years Kim's father will be twice as old as Kim.
Please help.

Answer 1

Philo did a fine job with the math. He deserves the points.

I would, however like to give you some explanation so you *understand* what the math is doing.

You have Kim's age, her dad's age and a third quantity.......time.

You know Kim's age and her dad's age, but you don't know, and need to know the time, which I'll call "t".

You take dad's age (39) and Kim's age (15) NOW, and then in some number of years, Kim's age will be twice that of the dad's.

Dad's age NOW......PLUS some number of years....

39 + t


This will equal TWICE Kim's age NOW PLUS some number of years.

2(15 + t)


You equate these two because this is what the problem says....'in how many years will this be TRUE'....that means equate the above two expressions.

39 + t = 2(15 + t)

Now....just look at Philo's math and you will have the whole story.

He gets the points!

Answer 2

39 + x = 2(15 + x)
39 + x = 30 + 2x
9 = x

Answer 3

2(15 + x) = 39 +x
39 + x = 30 + 2x
39 - 30 = 2x - x
9 = x