Originally Posted by

**Laurent** Your answer is the average number of tremors, not the most likely one.

The number of tremors to occur in a week can be approximately considered to be a Poisson random variable (that's in the title of the thread!) with mean $\displaystyle \lambda=3.5$ (using your computation). In other words, the probability of $\displaystyle k$ tremors in a week is about $\displaystyle e^{-\lambda}\frac{\lambda^k}{k!}$. Compute that value for $\displaystyle k=0,1,2,3,\ldots$ and conclude.