Le 21 pour vérifier s'il vous plaît

a) v = d/t soit t = d/v
montée : t = 10/15 = 2/3 h = 2/3 x60 = 40 min
descente t = 10/45 = 2/9 h = 2/9x60 = 40/3 min


b)Δt = 40+40/3 = 120/3+40/3 = 160/3min = 160/60x3 = 160/180 = 16/18 = 8/9 h

c) v = d/t
v = 20: 8/9
v = 22,5 km:h 
15km/h < 22,5 km/h < 45 km/h