Use speed = distance/time to find her initial speed = 100/5 = 20 mph
Hence speed in last part of the trip = v2 = 40 mph
Distance travelled in first part of trip = 100 miles
Distance travelled in second part of trip d2 = 300 - 100 = 200 miles.
Time taken for second part of trip = distance/speed = d2/v2 = 200/40 = 5 hours.