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→Calculating Count Rate
| style="height:20px; width:300px; text-align:center;" |'''1. Calculate the number of half lives which have occurred.'''
<math>17.5 \div ÷ 2.5 = 7</math>
n = 7 '''half lives'''
| style="height:20px; width:300px; text-align:center;" |'''1. Identify how many times the number of [[Unstable Isotope|unstable]] [[atom]]s has halved.'''
<math>25 \times 60 \div ÷ 300 = 5</math>
n = 5 '''half lives'''