$21.$ A subway train accelerates from $0$ to $60$ km$/$h in $20$ seconds. Calculate its acceleration$\mathrm{.}\mathrm{}$

$22.$ If a train departs from Station A at $9$:$00$ AM and arrives at Station B at $3$:$30$ PM$,$ how long was the journey?$\mathrm{}$

$23.$ A high$-$speed train covers a distance of $800$ kilometers in $4$ hours. What is its average speed in kilometers per hour?$\mathrm{}$

$24.$ Calculate the total weight of a freight train consisting of $10$ railcars, each carrying $50$ tons of cargo$\mathrm{.}\mathrm{}$

$25.$ If a rail track has a gradient of $1$ in $40,$ what is the rise in elevation for every $1$ kilometer of track?