t1=L/v弹=340/680 = 0.5s
子弹离开枪口后经0.5s击中靶
击中靶子声音的传回时间t2 = L/v声 = 340/340 = 1s
t1+t2=0.5+1 = 1.5s
从子弹离开枪口到小明听到子弹击中靶的声音经过1.5s.