Qur'anic style of stating figures

[Fajr]

As-salaamu `alaykum

It recently caught my attention that in quite a few places of the Qur'aan, when Allaah `azza wa jall mentions the number of something (especially on a particular occasion/matter), the exact number is not stated, but instead most of it is mentioned as well as either an addition of exclusion. For example:

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"And We appointed for Musa thirty nights and added (to the period) ten..." [al-A'raaf: 142]



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"And indeed We sent Nuh to his people, and he stayed among them a thousand years less fifty years..." [al-'Ankaboot: 14]



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"And they stayed in their Cave three hundred (solar) years, and add nine..." [al-Kahf: 25]


I just found this interesting and was wondering if anyone had more information on this - whether from tafseers (perhaps thematic), language or just general knowledge of the Qur'anic sciences. It could make a good discussion, wallaahu a'lam

Jazaakumullahu khayran

[aburasheed]

As-salaamu `alaykum

http://quran.islamicnetwork.com/images/7_142.gif
"And We appointed for Musa thirty nights and added (to the period) ten..." [al-A'raaf: 142]
I read (http://www.sacred-texts.com/isl/pro/mos19.htm) that Prophet Musa was commanded to fast for 30 days, after the completion of the 30 days, he rushed to brush his teeth eager to meet Allah, and Allah extended the fasting for another 10 days. I could not locate the source of the story, but if someone have the source please share it with us. It's probably from one of the Israliat.