In reply to Wil Treasure:
> I don't know why anyone would try to solve this with logarithms
If you had log and anti-log tabled to hand but no calculator it would make sense and would be the easiest way for sure. I’ve worked with someone who still had rote memorised log tables in their head from 40 years previously.
If I didn’t have a calculator, and didn’t have the necessary tables, I would partially solve it in my head to get r^6 = 1.3. Then as this isn’t much more than 1, I would take an estimate of r = 1 + (1.3 - 1) / 6 = 1.05 as my upper bound and half that amount above 1, r = 1.025, as my lower bound and do a few rounds of bisection to get the answer, with a lot of pen and paper multiplication.
Or perhaps I’d calculate the r^6 for 4 values between and including my bounds, plot them on a paper graph and draw a smooth curve, before reading off the answer.
Log tables definitely win in the absence of a calculator!
There’s presumably a pen and paper algorithm for computing fractional powers more directly, but I don’t know it. Edit: okay, I googled it - computers either use log/anti-log, bisection or Newton’s method. Two of which are embodied in my all time favourite WTF piece of code - https://en.m.wikipedia.org/wiki/Fast_inverse_square_root
Post edited at 21:29