Answer
$r = 60.0~m \Omega$
Work Step by Step
We can write an equation to express the relationship between $V_T$ and $i$:
$V_T = \mathscr{E}-ir$
When we input the data into the linear regression function of a calculator, the output is as follows:
$V_T = 13.6-0.0600~i$
From the parameters of the fit, we can see that the internal resistance is $~~r = 60.0~m \Omega$
