We have started with a count rate of 400 counts per minute. We need to find the time it takes for the count rate to halve. Half of 400 is 200. So we need to find 200 counts per minute on the graph and read the time it takes to reach this number. This is 1 hour on this graph. So it takes 1 hour for the count rate to halve, therefore the half-life is 1 hour.