A boat is tied up at a dock and bobs up and down. The vertical distance between its high and low point is 1.8 m and the cycle is repeated every 4s.

a) Determine a sine equation to model the vertical position, in metres, of the boat versus the time, in seconds.

b) Use your model to determine when, during each cycle, the boat is 0.5 above its mean position. Round your answers to the nearest hundreth of a second.