If a person lives on average 100 years in his rest frame, determine...

The proper time interval, which is the lifetime measured in the person's rest frame, is $\Delta t_0 = 100$ years.

The velocity of the spaceship relative to the Earth frame is $v = 0.6c$.

Using the time dilation formula, the lifetime measured in the Earth frame is given by:

$\Delta t = \dfrac{\Delta t_0}{\sqrt{1 - \dfrac{v^2}{c^2}}}$

Substituting the given values:

$\Delta t = \dfrac{100}{\sqrt{1 - (0.6)^2}}$

$\Delta t = \dfrac{100}{\sqrt{1 - 0.36}}$

$\Delta t = \dfrac{100}{\sqrt{0.64}}$

$\Delta t = \dfrac{100}{0.8} = 125 \text{ years}$