The difference in sound intensity between a motorcycle (90 dB) and an automobile (70 dB) passing by is 20 dB.
Using the formula for sound intensity in decibels, where a change of 10 dB represents a tenfold change in sound intensity:
20 dB = 10^(20/10)
20 dB is therefore 100 times more intense than 0 dB (the threshold of human hearing).
So the sound of a motorcycle passing by (90 dB) is 100 times more intense than the sound of an automobile passing by (70 dB).
Therefore, the answer is: one hundred times.