One question is like this One person climbs the mountain, climbs to the top of the mountain from the foot of the mountain, and then returns to the foot of the mountain along the original road. The average speed up the mountain is V1, and the average speed down the mountain is v2. What is the average speed and average speed of the round trip? How to express it? In addition, when the person goes up and down the mountain, the displacement is 0. When the position changes, the average speed is 0. Is that wrong? What's the difference between the two? I know that velocity is a vector and has direction of magnitude. Velocity is only a scalar, regardless of whether it is round-trip or not

One question is like this One person climbs the mountain, climbs to the top of the mountain from the foot of the mountain, and then returns to the foot of the mountain along the original road. The average speed up the mountain is V1, and the average speed down the mountain is v2. What is the average speed and average speed of the round trip? How to express it? In addition, when the person goes up and down the mountain, the displacement is 0. When the position changes, the average speed is 0. Is that wrong? What's the difference between the two? I know that velocity is a vector and has direction of magnitude. Velocity is only a scalar, regardless of whether it is round-trip or not

You are right. The average speed is measured by displacement, and the average speed is measured by distance. Up and down the mountain, the displacement is 0, and the distance is 2S
The average rate is V1V2 / (V1 + V2)
Assuming that the uphill time is t, the downhill time is v1t / V2, and the average rate is 2v1t / (T + v1t / V2) = 2v1v1 / (V1 + V2)