To represent Lindsay's guitar practice mathematically, we can define \( x \) as the number of weekdays she practiced.
From the problem, we know that she practices 50 minutes on Saturday, and on most weekdays she practices 30 minutes each day.
The total practice time for the weekdays would be \( 30x \) (since she practices 30 minutes each day for \( x \) days).
We can set up the equation to represent the total practice time:
\[ 30x + 50 = 140 \]
Now, we can rearrange this equation into the standard form \( px + q = r \):
\[ 30x + 50 = 140 \]
So, the equation in the required form is:
\[ 30x + 50 = 140 \]