Iodine-131 is used in hospitals - Edexcel - GCSE Physics - Question 4 - 2016 - Paper 1

To find the half-life of iodine-131, observe the graph and locate the point at which the mass declines to half of its original value. In this case, the initial mass is approximately 200 mg, so the half-life is when the mass reaches about 100 mg. By tracing a horizontal line from 100 mg to the graph, find the corresponding time on the x-axis, which indicates that the half-life is approximately 8 days.