Let's denote the number of weekdays Lindsay practiced as \( x \).
According to the information provided:
- She practices 30 minutes on most weekdays, so for \( x \) weekdays, she practiced \( 30x \) minutes.
- On Saturday, she practiced 50 minutes.
The total practice time for the week can be represented by the equation:
\[ 30x + 50 = 140 \]
This can be rewritten in the required form \( px + q = r \):
\[ 30x + 50 = 140 \]
So, the answer is:
\( 30x + 50 = 140 \)