I don't know how to start. L.C=0.1 s n=20 T=20 then for 20 oscillation=20 seconds time period for 20/20*1/10=0.1 time period (1-0.1)/0.1*100=90% (wrong)
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If we assume that the error in your measurement ("20 seconds") is 0.1 second ("the least count"), then you are measuring the time for 20 oscillations as 20.0 ±0.1 second, and the error is 1 part in 200 (0.1 in 20).
Because you are measuring 20 oscillations, the error of the stopwatch matters much less than it would if you measured just a single oscillation - in a sense you are dividing the error over 20 cycles, so it is as though the measurement of one cycle has an error of (0.1/20) sec/sec.
In reality, there will be errors in your starting and stopping the stopwatch, as well as in your ability to determine when the pendulum has complete a period (best done by seeing when the pendulum passes through zero... but that's probably not the point of this question).
Does that help?
Floris
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