φ(15)=15(1−13)(1−15)=15×23×45=8\varphi(15) = 15\left(1 - \tfrac{1}{3}\right)\left(1 - \tfrac{1}{5}\right) = 15 \times \tfrac{2}{3} \times \tfrac{4}{5} = 8φ(15)=15(1−31)(1−51)=15×32×54=8.