Bonjour,
a) 1 km = 1 000 m
1 000 m ÷ 14,12 s = 70,821529..... m/s
b) 1 h = 3 600 s
donc : vitesse = (1 000 ÷ 14,12) m/s × 3 600 s
= 254 957,507..... m/h
≅ 254, 958 km/h
