A18 gram sample of a substance that's used to detect explosives has a k-value of 0.125. find the substance's half-life, in days, and round to the nearest tenth.

For this problem, we use the formula for radioactive decay which is expressed as follows:

An = Aoe^-kt

where An is the amount left after time t, Ao is the initial amount and k is a constant. 

We calculate as follows:
A0 = 18
An = 18/2 = 9

k = 0.125

9 = 18 e^-0.125t
0.5 = e^-0.125t

t = 5.55 days