You think that was clever, look at this:-
18:25 And it is said they lived in the Cave for three hundred years. Others say, “Nay, add nine to that number.”
Why the additional nine years? Why not just say 309 and be done with it? The author of the Quran does not give us all the answers to life’s problems, He wants us to use our God given intellects to ponder over the verses and better ourselves; no gain without intellectual pain! The answer , my friends is simple once you have been told, 300 solar years is the same as 309 lunar years (do the maths yourself, lunar cycle of 29.25days, 12 cycles per year etc), hence the addition of nine years in the above to show the mathematical genius of the Author of the Quran.