Adam Clarke on Genesis 7:24

And the waters prevailed upon the earth a hundred and fifty days - The breaking up of the fountains of the great deep, and the raining forty days and nights, had raised the waters fifteen cubits above the highest mountains; after which forty days it appears to have continued at this height for one hundred and fifty days more. "So," says Dr. Lightfoot, "these two sums are to be reckoned distinct, and not the forty days included in the one hundred and fifty; so that when the one hundred and fifty days were ended, there were six months and ten days of the flood past." For an improvement of this awful judgment, see the conclusion of the following chapter, Gen 8:22 (note).