Answer :

rate = (150 miles)/(3 hrs) = 50 mph

so . . .

(50 mph)*(time) = 400 miles

*divide both sides by 50 mph

time = (400 miles)/(50 mph) = 8 hrs

time = 8 hrs
150 divided by 3 = 50 miles per hour. Therefor it would take them 8 hours to drive 400 miles. :)