Approximately 0.4373 seconds
Speed of sound at 20 degree C is roughly 343 meters/second
Speed of light = 3x10^8 or 300,000,000 meters/second
at 150.0 feet away you set it up as:
150.0/343 - 150/3x10^8
150.0/343 = 0.43731778
150/3x10^8 = 5x10^-7 or 0.0000005
Subtract 0.43731778 - 0.0000005
Answer is 0.43731728
Rounding would be approx. 0.4373