I think this should be the answer!!
The answer is 8 because for this question all you need to do is add 5 and 3
70dB is represented by 100cars in 60sec. Therefore if the number of cats is reduced by 3/4 in the same amount of time(60s), then the decibel level will reduce by 3/4 also. So 70dB * .25= 17.5dB