CAS8-S02:29 (ASM 17.2) - Why??

Stock prices follow GBM:
dS(t)= .11S(t)dt + .16S(t)dZ(t)

Calculate the probability of exceeding a continuously compounded annual return of 20% in a 2 year time period.

This is also exercise 17.2 in ASM.
According to the solution in the ASM Manual, we start off:
"For the associated ABM X(t) = ln S(t), X(2) - X(0) is a normally distributed random variable with...... "

Why do we do X(2) - X(0) instead of ln (S(2)/S(0)) ??


CAS8-S02:29 (ASM 17.2) - Why??