Scratch my back and I'll scratch yours
Possible interpretation: I will help you if you will help me. If you do me a favour I'll do one for you in return.Note: scratch (verb) = rub (a part of the body) with one's fingernails to stop an itch or for pleasure | Also known as: "You scratch my back and I'll scratch yours."
Origin: Possibly related to observations of animals like monkeys who scratch each other's backs because they can't scratch their own.
When Wanda said to Mary "scratch my back and I'll scratch yours", she meant