Exponential Growth and Decay

a) If a boat slows according to an exponential function, and its initial speed was 40km/h, what does the formula need to be if the boat is only going 10km/h and it has been 10 minutes since letting off the gas? Define the variables

b) How fast was the boat going 1 minute after letting off the gas?

c) How fast will it be going 15 minutes after letting off the gas?

d) How long does it take to cut the boats original 40km/h speed in half?

Im having trouble figuring out the formula needed to determine the answers. Any help would be great. Thanks