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 ...
No comments:
Post a Comment