Answer:
8 years.
Explanation:
let after x years father's age is 3 times that of Rahul.
31+x=3(5+x)
31+x=15+3x
31-15=3x-x
2x=16
x=16/2=8
8 yrs
3(5+x)=31+x
15+3x=31+x
x=2
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