יְשַׁלַּח־בָּ֨ם ׀ חֲר֬וֹן אַפּ֗וֹ עֶבְרָ֣ה וָזַ֣עַם וְצָרָ֑ה מִ֝שְׁלַ֗חַת מַלְאֲכֵ֥י רָעִֽים׃
ער האָט אָנגעשיקט אױף זײ דעם בראַנד פֿון זײַן צאָרן, כּעס און גרימצאָרן און לײַד, אַן אָנשיקעניש פֿון בײזע שלוחים.
Midrash Lekach Tov
Another explanation: Behold, twice ko is fifty (the numerical equivalent of ko being twenty-five). These are the fifty plagues with which the Egyptians were struck. How many were they struck with with the "finger" (Exodus 8:15)? Ten plagues. You can say from here that in Egypt, they were struck with fifty plagues and at the Sea, they were struck with 250 plagues. Since every plague that the Holy One, blessed be He, brought upon the Egyptians in Egypt was [composed] of five plagues. As it is stated (Psalms 78:49), "He sent upon them the fierceness of His anger, wrath, and fury, and trouble, a sending of messengers of evil."