Answer:
t = S / v    time = distance / speed
For Tom  t = 12 m / 4 mph = 3 hours to cover course
Tom will finish at 8:13 + 3 = 11:15
th = 12 m / 8mph = 1.5 hours to cover course
11:15 - 1.5 = 9:45 Â Â Â time Harriet should start to finish in 1.5 hours