An astronaut who was 30 years old left to explore the galaxy in 2020, on a spaceship

travelling at 2.5 x 108 m/s. He is expected to return in 2060. About how old will he appear to be?

52.1 years.

Explanation:

The time interval from 2020 to 2060 = 40 years. This will be the time as measured by an observer on earth.

For the person who is travelling at v = 2.5 * 10⁸ m/s,

t = 40 √ (1 - [ (2.5 * 10⁸) ² / (3.0 * 10⁸) ²] = 22.1 years

Actual age for his counterpart on earth would be 30 + 40 = 70 years,

But space travel makes the astronaut to age slowly (time dilation). So he appears to be 30 + 22.1 = 52.1 years old.

70 years old