When doing a division, to approximate the quotient you can approximate the numerator and denominator to close numbers so that the new operation is easire to calculate. For example, consider that we are going to divide 461/5. We can approximate the numerator to 460 since it is easier to divide 460 by 5.
since the approximation is limited to take closer numbers, the first digit of the estimated quotient should coincide with the first digit of the real quotient.