So, the average is found by taking two instants in time, say t1 and later t2, and you find out where it was at t2 minus where it was at t1 and divide by the time.

平均速度就是取两个时刻,比如t1和之后的t2,用t2时刻的位移,减去t1时刻的位移,再除以这段时间

耶鲁公开课 - 基础物理课程节选