#### Answer

5.3

#### 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, $a= 400$, $T=2.6$, and $B=3.1$.
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.
$R=log(\frac{400}{2.6})+3.1=log(153.85)+3.1=2.2+3.1=5.3$