The graph is not clear.
From the given graph Half-life is
The accompanying graph shows the rate of decrease of the count rate or % samples remaining with time.
For any particular radioisotope the % samples remaining and time will be different. However, the shape of the curve will be the same.
In order to measure the half-life from a graph
Read the % samples remaining for zero days.
In our graph it is 100.
Go down to half point where % sample remaining is 50 and draw a horizontal line meeting the curve.
Now draw a vertical line down from this point at the curve and read off the half-life where the vertical line meets the time axis.
From the given graph Half-life appears