First, install Linux (beta) by following the "Turn on Linux (Beta)" steps described here https://support.google.com/chromebook/answer/9145439?hl=en-GB
Then, copy-paste the following command into the Linux terminal (the black window that opens after you install Linux) and press Enter:
sudo apt update && sudo apt -y upgrade && sudo apt -y install r-base wget libnss3 && \
wget https://download1.rstudio.org/desktop/bionic/amd64/rstudio-1.4.1103-amd64.deb && \
sudo apt -y install ./rstudio-1.4.1103-amd64.deb
This will take about ~10 minutes, after which you will have fully functional R-Studio with R v3.5.