What is the conditional probability that Advertising $<=10$ for a US store with Population $=500?$

The dataset is named "Carseats" and can be found in R$.$

The tree is :

library$($ISLR$)$;

attach$($Carseats$)$;

library$($rpart$)$; library$($rpart$.$plot$)$;

tree $<-$ rpart$($Advertising ~ US $+$ Population, data $=$ Carseats$)$;

Please help using the ecdf function.

$$Use $2$ digits, like $0\mathrm{.}12$