Let:
x = Rachael's lowest test score
y = average score of the other tests
Rachael had an average score of 88 on 7 equally-weighted tests. so:
[tex]88=\frac{x+y}{7}[/tex]Her teacher decided to drop the lowest test grade. Rachael's average for
the 6 remaining tests is now 91. so:
[tex]\begin{gathered} 91=\frac{y}{6} \\ so\colon \\ y=6\cdot91 \\ y=546 \end{gathered}[/tex]Replace the valuel of y into the first equation:
[tex]88=\frac{x+546}{7}[/tex]solve for x:
[tex]\begin{gathered} 88\cdot7=x+546 \\ 616=x+546 \\ x=616-546 \\ x=70 \end{gathered}[/tex]