Example the 5,521×71 HOW DO YOU ESTIMATE BY FIRST ROUNDING EAH NUMBER SO THAT IT HAS ONLY 1 NON ZERO DIGIT

Question

Need help on this formula how do you estimate by first rounding each number so that it has only 1 non zero Digit example 5,521×71

Answer 1

6000 x70 = 420,000

which is off by 18,009

which is off by 18,009/391,991 = 0.046

= about 4.6%

a reasonable margin of error and approximation