Answer:
156.67 m/s
0.45676 times the speed of sound
Explanation:
Distance from the ground = 23.5 km = 23500 m
Time taken by the blast waves to reach the ground = 
Spedd of the wave would be

The velocity of the blast wave is 156.67 m/s
v = Velocity of sound = 343 m/s

The blast wave is 0.45676 times the speed of sound
It would be 78 becuase the 2 percent of 64 is many of energy lost