With rest start, no air and constant g = 10 m/s^2, distance fallen is y = gt^2/2 and final speed is v = gt. At two seconds, distance is 20 m and average speed over the interval is 10 m/s.
Worked example
The first second contributes 5 m. The next contributes 20 - 5 = 15 m. Equal time intervals have unequal distances because speed is increasing.
With no air and constant g of 10 meters per second squared, an object released from rest falls 5 meters in its first second and 15 more meters in its second second. Total distance is 20 meters.
