You can use dimensional analysis. \(\frac{30 km}{1 s} \frac{1000 m}{1 km} \frac{100 cm}{1 m} \frac{1 in}{2.54 cm} \frac{1 ft}{12 in} \frac{1 mi}{5280 ft} \frac{60 s}{1 min} \frac{60 min}{1 hr}=\)\(\frac{10800000000 mi}{78638.4 hr} or \frac{137337.484 mi}{1 hr}\)