Shouldn't he say 3/5, not 4/5? 2000 / 1.4 = 1428.57...
Marc Diamond, Providence, RI
Dear Marc,
The Maharshal (Chochmas Shlomo printed with the Maharsha) had a text written in Rashi that reads 4/5. He explains that Rashi used this number not as the exact side of the square which makes a diagonal of 2000. Rashi himself afterwards calculates 1428 and 2/5, without the added fraction, but as the amount that is "missing". It seems he is referring to the 4/5 missing from 2000 after adding 2/5 to 1428. 1428+571.2=1999.2. The Maharshal says afterwards that 1428 3/5 is sufficient as you noticed.
The Bach is just quoting Rashi as he is brought down by the Maharshal. (The Maharsha has a slightly different explanation).
At the end of the Bach it is written in brackets " and so it in the Venetzia edition". However, in the first edition of Venetzia we found no fraction at all added to 1428.
All the best,
Reuven Weiner