answer: 200 miles traveled for 4 hours at speed 50 mph

distance = speed times time

d=st miles = mph x hours = miles/hour x hours

t= time for train at 40 mph 9-t= time for train at 50mph

d = d1 + d2

= s1t + s2t

= 40t +50(9-t)

400 = 40t + 450 -50t

10t = 50

t =50/10 =5 hours at 40 mph

that's 5 x 40 = 200 miles at 40 mph, with 400-200= 200 miles at 50 mph

t = 5 hours, 9-5 = 4 hours at 50 mph

4 hours at 50 mph = 4x50 = 200 miles

5 x 40 mph + 4 x 50 mph = 200 + 200 = 400 miles