Question 884870: after five year, the age of a father will be thrice the age of his son, whereas five years ago, he was seven times old as his son was. What is father's present age?
Answer by JulietG(1812)   (Show Source):
You can put this solution on YOUR website! F + 5 = 3(S+5)
F - 5 = 7(S-5)
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The easiest way to do this problem is to isolate S. We'll do that by finding the value of F and substituting.
F + 5 = 3(S+5)
Subtract 5 from each side.
F = 3S + 15 - 5
F = 3S + 10
Now take that value of F and put it into the second equation.
F - 5 = 7(S-5)
(3S+10)-5=7S-35
3S+5 = 7S - 35
Subtract 3S from each side
5 = 4S - 35
Add 35 to each side
40 = 4S
Divide each side by 4
10 = S
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If the son is 10, then in five years, the son will be 15. The father will then be 3 times that, or 45. If he is 45 in 5 years, then he is 40 now.
Let's check by working the second half.
"five years ago, he was seven times old as his son was. "
Five years ago, the son was 5. Five years ago, the father was 35.
That is indeed seven times the age.
Success!
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