Solution

Let's use the formula from the Lorentz transformations:

$$t' = \frac{t_0}{\sqrt{1 - (v/c)^2}} \implies \frac{t'}{t_0} = \frac{1}{\sqrt{1 - (v/c)^2}}$$

$$\frac{t'}{t_0} = \frac{5}{3}$$