AL phy 問題 唔該幫幫手 要解題(即係step) 內有Ans

i think there should be one more condition, it needs to state that what is the final position for the boat,as its final position affects the average speed, i thnik the final position is the origial position i this question.

average speed = total distance travelled / total time used....(1)
downward speed 8ms-1
upward speed 4ms-1
suppose travelled in the downward motion spends x units of time, then for the upward motion, te time is 2x units.

total distance travelled: 8*x 4*2x = 16x
total time: x + 2x = 3x
so By (1): average speed = 16x / 3x = 5.33ms-1(3 sign.fig.)