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Dave's current age is the age of his favourite book (which is 14 years old) plus one third of his age.
How old will he be when his favourite book is twice its current age?
Puzzle Copyright © Kevin Stone
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Answer
He will be 35 in 14 years.
Reasoning
He is currently 21.
If X is his current age, then:
X = 14 + X
———
3
so
2X = 14
———
3
and
2X = 42
So
X = 21
He is currently 21 and his favourite book is currently 14 years old, when it is 28 years old (double its age) he will be 35.