Solution
= Solution
Since $136=8\cdot17$ and the factors are coprime,
$$
(\mathbb Z/136\mathbb Z)^\times\cong
(\mathbb Z/8\mathbb Z)^\times\times(\mathbb Z/17\mathbb Z)^\times.
$$
Hence
$$
\boxed{|G|=\phi(136)=\phi(8)\phi(17)=4\cdot16=64.}
$$
= Solution
Since $136=8\cdot17$ and the factors are coprime,
$$
(\mathbb Z/136\mathbb Z)^\times\cong
(\mathbb Z/8\mathbb Z)^\times\times(\mathbb Z/17\mathbb Z)^\times.
$$
Hence
$$
\boxed{|G|=\phi(136)=\phi(8)\phi(17)=4\cdot16=64.}
$$