17.525 hours
First, determine how many half lives the substance has undergone by taking the logarithm to base 2 of the amount left. So log(0.0625)/log(2)
= -1.204119983/0.301029996
= -4
So the substance has undergone exactly 4 half lives. This makes sense since 1/2^4 = 1/16 = 0.0625.
Since 4 half lives took 70.1 hours, the duration of 1 half-life will be 70.1/4 = 17.525 hours.
