CAIE A-Level Physics A2 23.2.2 Radioactive Decay Is Both Spontaneous and Random Topic Practice

Carbon-11 is radioactive and decays by $\beta^{+}$emission to form boron-11. Carbon-11 has a half-life of 20 minutes. Boron-11 is stable.

Explain, with reference to the random nature of radioactive decay, why the activity of the carbon-11 sample in (b) decreases with time.