suppose you put one bacterium on a petri plate containing a suitable medium at a favorable temperature. If after 6 hours you found there to be 40 bacterium. What would be its exact double time to the nearest minute?

40 = 1(2)^(6/k) , where k is the doubling time in hours

log 40 = log (2^(6/k) )
log 40 = (6/k) log 2
6/k = log 40/ log 2

k = 6log2/log40 or log 64/log40

since you asked for the "exact" doubling time, that is your answer