I have a plot which contain large number of points. I want to find an equation that calculates the percentage of a certain number of these points $(x,y)$, the ones having $x>5$ and $y>80$.
In other words, 100 $\times$ (all points higher than (5 in x-axis and 80 in y-axis)) / (all points in the plot).
How can I find this equation? Thank you!
You are essentially there. Often we put vertical bars around a set to indicate the number of elements, so if your set of points is set $A$ you could write $\frac {|\{(x,y)|(x,y) \in A \land x \gt 5 \land y \gt 80\}|}{|A|}$ where the numerator is in set-builder notation. The vertical bar says "take all the ordered pairs $(x,y)$ that satisfy all this". By the way, this is an expression, not an equation, as it isn't shown as equal to anything.