$解：(1)t_{ }=\frac {s_{ }}{v_{ }}=\frac {{ (6800+200) }\ \text {m}}{{ \frac {72}{3.6} }\ \text {m/s}}=350\ \text {s}$
$(2)t_{ }'=\frac {s_{ }'}{v_{ }}=\frac {{ (6800-200) }\ \text {m}}{{ \frac {72}{3.6} }\ \text {m/s}}=330\ \text {s}$
$(3)t_{ }''=\frac {s_{ }''}{v_{ }}=\frac {{ 6800 }\ \text {m}}{{ \frac {72}{3.6} }\ \text {m/s}}=340\ \text {s}$
