Graphical comment: @whuber's picture.

## a million points for 2-3 digits of accuracy
set.seed(319)
X = rnorm(10^6); Y = rnorm(10^6)
mean(Y > 3*X & Y > 0)
[1] 0.300751
mean(Y > 3*X & Y > 0)/mean(Y > 0)
[1] 0.6027038

## Fewer points for figure
x = X[1:50000];  y= Y[1:50000]
plot(x,y, pch=".")
 abline(a = 0, b = 3, col="green", lwd=2)
 abline(h=0, col="red", lwd=2)