[tex]\huge\boxed{18\ \text{points}}[/tex]
We'll represent Louise's, Tammy's, Delores's, and Sheryl's point values with [tex]x[/tex], [tex]x+1[/tex], [tex]x+2[/tex], and [tex]x+3[/tex] respectively since each one is 1 point more than the last.
Add all of these values up and set it all equal to [tex]78[/tex].
[tex]x+x+1+x+2+x+3=78[/tex]
Now, simplify.
[tex]4x+6=78[/tex]
Subtract [tex]6[/tex] on both sides.
[tex]\begin{aligned}4x+6-6&=78-6\\4x&=72\end{aligned}[/tex]
Divide both sides by [tex]4[/tex].
[tex]\begin{aligned}\frac{4x}{4}&=\frac{72}{4}\\x&=\boxed{18}\end{aligned}[/tex]
Since Louise's score is [tex]x[/tex], the answer is [tex]18[/tex].
To verify our answer, add the point totals [tex]18[/tex], [tex]19[/tex], [tex]20[/tex], and [tex]21[/tex].
This equals [tex]78[/tex], so we can be sure the answer is correct.