我想问一道英文的数学题!
Johnleaveshomeandstartwalkingat4km/htohisfriend'shouse.Halfahourlater,hissisterSusanleavesthehouseonabicycleandheadsinthesamedirectionat12km/h.HowfarawayaretheyfromthehousewhenSusancatchesuptoJohn,andhowlongafterJohnfistleftthehouse?