(a) Precision is given by the least count of the instrument.
For 20 oscillations, precision = 0.1 s
For 1 oscillation, precision = 0.005 s.
(b) Average time t = (39.6 + 39.9 + 39.5/3)s = 39.6s
period = 39.6/20 = 1.98 s
Max. observed error = (1.995 - 1.980)s = 0.015s