Answer:
Could you please check the numbers used in this problem. I'm unclear why the father is aging so much faster than the son. At one point, the father is 5 times older. As they both age at the same rate (my assumption), the multiplier should drop, not increase from 5 to 7.
The phrase "The son's age is one fifth of the father's age in 7 years" might be missing a comma, or period.
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Assume the son is 1 year old at present. The father would be 5 (son is 1/5 of the father). Add seven years to both, the ratio of father to son would be 12/8, or 1.5 times. The ration should keep dropping with time, not increasing. That is, unless a negative age is involved.
Explanation:
If we accept the written information, we obtain a negative age for both the son and father:
Let S and F by the son's and father's ages, respectively.
We are told that S = (1/5)F as of today. [son's age is one fifth of the father's age]
We learn that in 7 years the father will be 7 times that of the son:
F+7 = 7(S+7)
Since S = (1/5)F, we can substitute that is the second equation:
F+7 = 7(S+7)
F+7 = 7((1/5)F+7)
F+7 = (7/5)F + 49
-(2/5)F = 42
F = -122.5
S = -24.5
I may be misinterpreting the problem.