The default memory organization if nothing is specified- simultaneous access mem. org.
Therefore, in write-through we would use simultaneous access memory organization.
Note- In write-back, since the simultaneous access mem. org. doesn't support, therefore use the strict hierarchical access mem. org.
Therefore, Answer to the question-
tavg = 0.85*(0.92*10 + 0.08*100) + 0.15*100 = 29.62 ns.