50 Days

תספרו חמשים יום                                                                                                   

The Rosh explains that even though it says fifty days it means forty-nine days but the Torah rounds it off to the nearest

ten the same way as when the Torah tells us to give forty Malkos it is actually thirty-nine and rounded off to the nearest 10

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We also find the Torah says they came down to Mitzrayim with 70 people and actually there were only 69. 

(  רא"ש סימן מ' ערבי פסחים)

In Ta'anis 29a The Pasuk says the Meraglim returned after 40 days the Gemoro asks it was only 39 days

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In  Kiddushin 38a on the Pasuk that they ate the Manna for 40years the Gemoro asks, it was 40 years minus 30 days.

In Sanhedrin 107b the Gemoro asks on the Pasuk that Dovid Hamelech was king for 7years (Chevron) + 33 years

(Yerushalayim) and the other Pasuk it says He was king in Chevron 7 1/2 years.

According to the Rosh, why doesn't the Gemoro answer in these 3 cases, that the Pasuk rounds it

off to the nearest 10?