Sunday, September 2, 2018

elementary number theory - Leading Digit of 24242


How could I solve this problem?



Find the first digit of 24242 without using a calculator.



I know how to find the last digit with modular arithmetic, but I can't use that here.


Answer




This is probably not the answer you are looking for, and wil probably only be appreciated by people of my age ...


I can still remember from school days that log102=0,30102999 (I always thought it was noteworthy that it is so close to 0.30103) - people who went to school in the 1950's can probably recall using logs to base 10 for lots of tedious calculations.


You can then do the multiplication by 4242 without a calculator, and get the fractional part (=x, say, but you are likely to need a calculator to find out the first digit of 10x, unless you have also memorised log2,log3,,log9 (I can't!)


Edit:


With a bit more digging in the recesses of my memory, I can just recall that log3 is something like 0.477, so log9=2log3=0.954, so that should do it ...


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