Answer
After the $250~s$ firing, the mass of the rocket is $~~1.35\times 10^5~kg$
Work Step by Step
We can find the mass of fuel that is consumed in 250 seconds:
$m = (480~kg/s)(250~s) = 1.20\times 10^5~kg$
We can find the final mass of the rocket:
$M_f = M_i-m$
$M_f = (2.55\times 10^5~kg)-(1.20\times 10^5~kg)$
$M_f = 1.35\times 10^5~kg$
After the $250~s$ firing, the mass of the rocket is $~~1.35\times 10^5~kg$
