Question 991916
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S=son's age; F=father's age=4S 
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F+5=3(S+5) . Substitute for F.
4S+5=3S+15
S=10
ANSWER 1: The son is 10 years old.
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F=4S=4(10)=40
ANSWER 2: The father is 40 years old.
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CHECK:
F+5=3(S+5)
40+5=3(10+5)
45=3(15)
45=45