A bus made a roundtrip journey from Madison towards Chicago, which is 240 km away. In the onward journey, the bus divided the journey time into 3 equal blocks. Started at 50 kph, it took stoppages after every block and increased its speed by 10 kph after every stoppage. At Chicago, it took a halt of 7 hrs 44 minutes. The return journey was divided into two equidistant blocks, covered at 45 kph and 75 kph respectively. Find by what amount the average speed of the whole journey of the bus is less than the sum of the average speeds of onward and return journey;

A. 75 kph
B. 86.25 kph
C. 97.5 kph
D. 105 kph
E. 120 kph

Answer: B

Source: e-GMAT