#### Answer

4.2

#### Work Step by Step

We are given that the formula $R=log(\frac{a}{T})+B$ can be used to find the intensity R of an earthquake on the Richter scale (where a is the amplitude in micrometers, T is the time in seconds between waves, and B is a constant).
In this case, $a= 200$, $T=1.6$, and $B=2.1$.
We can plug in these values to find R. Recall that $log(\frac{a}{T})$ is written in the form of the common logarithm and has a base of 10.
$R=log(\frac{200}{1.6})+2.1=log(125)+2.1=2.1+2.1=4.2$