There was a post about Heavy Division Shortcuts a while ago, but it involved 10 ^ exponent, and this question does not. On one of the FDP flashcards, the question says:
1,863,471 / 626,502 = (estimate)
a) 3
b) 4
c) 5
d) 30
e) 35
The answer says that you can basically cross out all the numbers besides the first 18 in the numerator and the first 6 in the denominator. The flashcard has no explanation as to why this is okay, and I have never seen this before. Can you please explain? It's flashcard #6. Thank you!