There are certain other requirements laid down such as the amount of instrument flying, and you do of course have to cover the whole sylabus, but different people will cover different bits at different speeds. It would be a bit rotten if you
had to do say 30 hours dual and 15 solo and covered it all in 45 hours having done 33 hours dual and 12 solo - or even worse 18 hours solo and 27 dual. The 10 hour unspecified allows a little bit of flex - but most people take more than 45 hours anyway.
