Answer:
Liam has finished 18 lessons
Explanation:
Let
x -----> the number of lessons finished by Liam
we know that
If Liam has finished the same percent of lessons from his piano book than Jenny
then
the finished lesson and the number of lessons are proportional
Using proportion
![(12)/(20)=(x)/(30)\\\\x=12*30/20\\ \\x=18\ finished\ lessons](https://img.qammunity.org/2020/formulas/mathematics/middle-school/kw5wdxkgtn6y818bdhffj7mvhkfyn5lzmb.png)