The deck of a bridge is suspended 275 feet above a river. If a pebble falls off the side of the bridge, the height, in feet, of the pebble above the water surface after t seconds is given by y − 275 - 16t².

(a) Find the average velocity of the pebble for the time period beginning when t = 4 and lasting

(i) 0.1 seconds 

(ii) 0.05 seconds 

(iii) 0.01 seconds

(b) Estimate the instantaneous velocity of the pebble after 4 seconds.