Answer:
steps below
Step-by-step explanation:
upstream speed = u-v upstream trip takes 40 minutes (2/3 hour)
downstream speed = u+v downstream trip takes 25 minutes (5/12 hour)
(u-v) * 2/3 = 10 2u-2v = 30 ... (1)
(u+v) * 5/12 = 10 u+v = 24 2u+2v = 48 ... (2)
(1)+(2): 4u = 78 u = 19.5 km/h
(2)-(1): 4v = 18 v = 4.5 km/h
