[tex] \dfrac{p}{q}=0.12\overline{54}\\\\\\\dfrac{p}{q}=0.1254545454\ldots\qquad|\cdot100\\\\\\100\cdot\dfrac{p}{q}=12.54545454\ldots\\\\\\100\cdot\dfrac{p}{q}=12+0.54545454\ldots [/tex]
Now let:
[tex] x=0.54545454\ldots\qquad|\cdot100\\\\100x=54.54545454\ldots\\\\100x=54+0.54545454\ldots\\\\100x=54+x\\\\99x=54\qquad|:99\\\\\\\boxed{x=\dfrac{54}{99}} [/tex]
So:
[tex] 100\cdot\dfrac{p}{q}=12+0.54545454\ldots\\\\\\100\cdot\dfrac{p}{q}=12+x\\\\\\100\cdot\dfrac{p}{q}=12+\dfrac{54}{99}\\\\\\100\cdot\dfrac{p}{q}=12\cdot\dfrac{99}{99}+\dfrac{54}{99}\\\\\\100\cdot\dfrac{p}{q}=\dfrac{1188}{99}+\dfrac{54}{99}\\\\\\100\cdot\dfrac{p}{q}=\dfrac{1242}{99}\\\\\\100\cdot\dfrac{p}{q}=\dfrac{138}{11}\qquad\Big|:100\\\\\\\dfrac{p}{q}=\dfrac{138}{11\cdot100}\\\\\\\dfrac{p}{q}=\dfrac{138}{1100}\\\\\\\boxed{\dfrac{p}{q}=\dfrac{69}{550}} [/tex]