Answer
158 times stronger
Work Step by Step
The relative strength is equal to $$=10^{M_2-M_1}$$ Plugging in values of $M_1=4.2$ and $M_2=6.4$ yields $$=10^{6.4-4.2}=10^{2.2}=158$$ Therefore, the waves are 158 times stronger.
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