1)
Fifteen Amos above what did the water rise?
1.
Rashi: After rising to equal the top of the mountains, the water rose by another fifteen Amos. 1
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Gur Aryeh: The verse (7:19) already stated that "all of the tall mountains were covered," and there certainly were mountains taller than 15 Amos. Rather, the entire earth with its mountains must have already been covered, and they were now covered by an additional 15 Amos. See Gur Aryeh.