Half Life Graph Calculation with Count Correction - GCSE Physics

A graph of Count Rate vs Time can show the decay of a radioactive isotope. *** Please note mistake at 4:18 - should be days, not hours! :)
The half-life of the radioisotope can be calculated from this graph, but the background radiation will first need to be subtracted.
This video takes you through the calculation of half life using a background count correction.
The original graph show the count rate of the sample, but ALSO includes the background radiation count.
We can obtain the 'pure' count rate by subtracting the background count. So how do we find the background count?
The background count is 20cpm because that is what's left after the sample has decayed (i.e. from 8 hours onwards).
So the first point on the new graph is 100 - 20 = 80cpm at t=0
The second point on the new graph is 60-20 = 40cpm at t=1
and so on...
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