logx(625)-logx(25)=2
By the change of base theorem
log 625 / log x - log 25 / log x = 2
log 25^2 / log x - log 25 / log x = log 100
[ 2log 25 - log 25 ] / log x = log 100
log 25 / log x = log 100
log x = log 25 / log 100
10^(log 25 / log 100) = x
10^( [log 25 ] / 2) = x
