Answer
$\frac{6}{7}$
Work Step by Step
We know that $probability=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}$
The total number of students (the number of all outcomes) is $350$
By using the table, there are $140$ low income and $160$ middle income families, thus there are $140+160=300$ non-high income families.
Hence the probability here is: $\frac{300}{350}=\frac{6}{7}$