4 hours
using the formula
distance = speed × time (t)
the Ford will have travelled 3 × 40 = 120 miles before the Chevy leaves
the Ford's distance can be modelled as
distance = 40t + 120
the Chevy's distance can be modelled as
distance = 70t
equating the right sides and solve for t
70 t = 40t + 120 ( subtract 40t from both sides )
30t = 120 ( divide both sides by 30 )
t = 4
the Chevy catches up with the Ford after 4 hours
