Akash travels from D to S at the speed of 15 km/h , from S to X at 30 km/h and from X to Z at 45 km/h.  If DS = SX = XZ  = 90 (in km) , then the average speed of Akash for the whole journey is:

24\(\frac{6}{11}\) km/h

Let, DS (D₁) = SX (D₂) = XZ (D₃) = 90 km

T₁ =  \(\frac{D_1}{S_1}\) = \(\frac{90}{15}\) = 6 hours

Similarly,

T₂ =  \(\frac{90}{30}\) = 3 hours

T₃ =  \(\frac{90}{45}\) = 2 hours

Total time = T₁ + T₂ + T₃ = 6 + 3 + 2 = 11 hours

Total distance = D₁ + D₂ + D₃  = 90 + 90 + 90 = 270 km

Average speed =\(\frac{total\;distance}{total\;time}\)

                      = \(\frac{270}{11}\) = 24 \(\frac{6}{11}\) km/h