A18 gram sample of a substance that's used to sterilize surgical instruments has a k-value of 0.1226. n = noe -kt no=invalid mass (at time to =0. n = mass at time t. k=a positive constant that depends on the substance itself and on the units used to measure time. t=time, in days. what is the substances half life, in days rounded to the nearest tenth.

The half-life is defined to be the time it takes for any specified property, in this case, mass, to be half of its original value. Thus, given the equation:
N = No*e^-kt, with N = 9, No = 18, and k = 0.1226, the value of t, in days, can be determined. Solving the equation, the half-life of the substance, or the time it takes for the sample to be 9 grams is 5.7 days. 

Hmm confusing don't understand