Answer:
<em>she'll be 42.3 years old</em>
<em></em>
Explanation:
The year she left is 2020
she returns in 2040
the time observed to have passed by on earth is 40 years
she traveled at a speed of 2.5 x 10^8 m/s
From relativity, the astronaut will experience a time dilation and will age more slowly.
the time that is observed to pass on her ship will be gotten from
t' =
where t is the time that is observed to pass by an earthly observer
v is the speed of her ship
c is the speed of light = 3 x 10^8 m/s
v/c = 2.5/3 = 0.83
t' = = 40 x 0.5578 = <em>22.3 years</em> would have passed for her
This means that she'll be 20 + 22.3 = <em>42.3 years old</em>