In 5 years, a person will be twice as old as they were 3 years ago. What is the person's current age?

Let $x$ be the person's current age. In $5$ years the age is $x+5$, and $3$ years ago it was $x-3$. The condition says $x+5=2(x-3)$, so $x+5=2x-6$, which gives $x=11$, choice G. Check: in $5$ years the person is $16$, and $3$ years ago the person was $8$, and $16=2\cdot8$. Choice F ($8$) is the age $3$ years ago. Choice J ($16$) is the age in $5$ years. Choice H ($13$) gives $18$ and $10$, and $18\ne20$, so it fails the condition.