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November 29, 2015
Posted by **lele** on Thursday, April 30, 2009 at 10:10pm.

- 7th grade algebra -
**RickP**, Friday, May 1, 2009 at 2:50amLet f = father's age and s = son's age.

"the father's age is three times his son's age"

f = 3s

"the difference between a father's age and his son's age"

f - s

If we want to know f - s, and have f = 3s, then we need to subtract s from the left side. And what we do to one side of an equation we must to the other side as well, so we subtract s from both sides.

(f) - s = (3s) - s

f - s = 2s

Now try some numbers for the son's age.

If the son is 14 (s = 14), then the difference between the father's and son's age would be 2s = 2(14) = 28. Does that work? The father is 3x the age of the son, so if the son is 14, the father is 3 x 14 = 42. And yep, 42 - 14 = 28.

If the son is 17 (s = 17), then the difference between the father's and son's age would be 2s = 2(17) = 34. Does that work? The father is 3x the age of the son, so if the son is 17, the father is 3 x 17 = 51. And yep, 51 - 17 = 34.