We know, Speed = Distance / Time
Just apply the values into the formula in every part:
A) Distance = 280 cm = 2.80 m
time = 7 s
So, speed = 2.80 / 7 = 0.4 m/s
B) Distance = 1000 miles
time = 4 h
So, speed = 1000 /4 = 250 mph
C) Distance = 12500 m
time = 50 min = 3000 s
So, speed = 12500 / 3000 = 4.17 m/s
Hope this helps!