Answer:
10.8 km/h to the nearest tenth.
Explanation:
Let x be the average with no wind.
Speed = distance / time
With the wind:
x + 3 = 15 / t A where t = time in hours for the first half of journey
Against the wind:
x - 3 = 15/(3 - t) B
Subtract A - B:
6 = 15/t - 15(3 - t)
6t(3 - t) = 15(3 - t) - 15t
18t - 6t^2 = 45 - 30t
6t^2 - 48t + 45 = 0
2t^2 - 16t + 15 = 0
t = - 1.085
So average speed (from equation B)
x = 15/ (3 - 1.085) + 3
= 10.8 km/h to the nearest tenth.