數學問題 40分

According to a research of oceanography, for every metre that a diver does down, the light intensity will decrease 2.5%. How many metres should a diver go down such that

a.the light intensity is one fourth of that at the water surface?
b.the light intensity is 1% of that at the water surface?






The magnitude of an earthquake occured in region A was 7, while a smaller earthquake occured in the nearby city B at the same time. the amplitude of the earthquake in city B was 1/3000 that of city A. Find the magnitude of the earthquake in city B correct to 1 decimal place.




Thank you