Who knows fifty?
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Fifty are the cubits' width of the Heichal/Ohel Moed portion of the Mishkan.
If you go to Shiloh, they have what they very strongly believe to be the remainder of the stone wall that supported the Mishkan there (instead of the wooden beams used in the desert). I measured the width, heel-to-toe, and got almost exactly 75 feet, which would fit with the definition of the cubit as 18 inches.
Fifty are the letters on the Cohen's stones.
(It fits the original song rhythm. The shoulder-stones on the Ephod each had 25 letters.)
Fifty shekels (worth about $460, given today's silver prices) is the erech ("endowment value") assigned to a male during his main working years. R' Hirsch explains:
50 = ((3 = family role) + (2 = communal role)) x (10 = completion)
Shavuos is 50 days after Pesach begins. (Vayikra 23:16)
50 are the shaarei binah - the levels of comprehension of G-dliness.
Moshe was granted 49 of these during his lifetime (Rosh Hashanah 21b), and at his death he was granted the 50th - the ultimate level of closeness to Hashem. This is hinted at by the name of the place of his passing, Mount Nevo, which parses out to נ' בו - "there are 50 in him." (Arizal to Deut. 3:26)
רַבִּי יוֹסֵי הַגְּלִילִי אוֹמֵר: מִנַּיִן אַתָּה אוֹמֵר שֶׁלָּקוּ הַמִּצְרִים בְּמִצְרַיִם עֶשֶׂר מַכּוֹת וְעַל הַיָּם לָקוּ חֲמִשִּׁים מַכּוֹת?
50 is the price in Shekels that David payed for Aravna's threshing floor. (II Samuel 24:24)
The Teivah was 50 cubits wide. (Bereishis 6:15)
A Purim Torah - Where does Noach come into the Megillah? Haman went to look for a tree that's 50 amos high... he searched and searched until he came to Noach's Teivah
1 Kings 18:13
הֲלֹֽא־הֻגַּ֤ד לַֽאדֹנִי֙ אֵ֣ת אֲשֶׁר־עָשִׂ֔יתִי בַּהֲרֹ֣ג אִיזֶ֔בֶל אֵ֖ת נְבִיאֵ֣י יְהוָ֑ה וָאַחְבִּא֩ מִנְּבִיאֵ֨י יְהוָ֜ה מֵ֣אָה אִ֗ישׁ חֲמִשִּׁ֨ים חֲמִשִּׁ֥ים אִישׁ֙ בַּמְּעָרָ֔ה וָאֲכַלְכְּלֵ֖ם לֶ֥חֶם וָמָֽיִם׃ My lord has surely been told what I did when Jezebel was killing the prophets of the LORD, how I hid a hundred of the prophets of the LORD, fifty men to a cave, and provided them with food and drink.