Answer
about 1.3
Work Step by Step
The Richter scale measures the intensity of earthquakes: $M=\displaystyle \log\frac{I}{S}$
where I is the intensity of the earthquake (amplitude in cm of a seizmograph measured 100 km from the epicenter),
and S is the intensity of a "standard" earthquake (amplitude of 1 micron$=10^{-4}$cm)
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Let $I_{1}$ be the intensity of the less intense earthquake,
and $I_{2}=20I_{1} $be the intensity of the more intense earthquake.
Then,for the magnitudes, we have
$M_{2}-M_{1}=\displaystyle \log\frac{I_{2}}{S}-\log\frac{I_{1}}{S}=$
... quotient rule for logarithms ...
$=\displaystyle \log(\frac{I_{2}/S}{I_{1}/S})=\log(\frac{20I_{1}}{I_{1}})=\log 20\approx 1.3$
The more intense earthquake has s Richter scale reading
about 1.3 greater than the less intense earthquake
