How do you convert -220 degrees to radians without a calculator?

You probably know that ##180°## corresponds to ##pi## radians;
You can build the proportion:
##180°:pi=()°:(rad)##
Set ##()°=-220°## and solve for ##(rad)##;
you’ll get:
##(rad)=-220°pi/(180°)=-22°pi/(18°)=-11/9pi##