we count 49 not 50 days

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

​

​

 . 

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)

n Ta'anis 29a The Pasuk says the

​I​

Meraglim returned after 40 days the Gemoro

​.​

asks it was only 39 days

​​

In  Kidushin 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 (Yerushlayim)
and the other Pasuk it says He was king in
Chevron 71/2 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?

​???​

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