?$12\div \frac {1}{8000000}=96000000(\mathrm {cm})$?
?$96000000\ \mathrm {cm} = 960\ \mathrm {km}$?
?$960\div 5 = 192(\mathrm {km/h})$?
甲車:?$192×\frac {5}{5 + 7}=80(\mathrm {km/h})$?
乙車:?$192×\frac {7}{5 + 7}=112(\mathrm {km/h})$?
答:甲車速度為?$80\ \mathrm {km/h},$?乙車速度為?$112\ \mathrm {km/h}。$?