Assuming that the elapsed time is y, PE= 10y and QD=5y, then 800+5y/2+5y-400= 15/2 y, 3qc/2-am = 3 (200+5y)/2-65438.

Let the elapsed time be y,

Then PE= 10y, QD=5y,

So PQ point is [0-(-800)]+10y-5y = 800+5y,

Half is 800+5y/2,

So the AM point is: 800+5y/2+5y-400= 15/2 y,

And QC=200+5y,

Therefore, 3qc/2-am = 3 (200+5y)/2-15/2y = 300 is a constant value.