[vivegi]

Peter: I know the product and it is p.q Sandy: I know the sum and it is p + q Peter and Sandy (in unison): Got it!

another variant:

Peter: If I divide by 4, I have remainder x Sandy: If I divide by 4, I have remainder y Peter: If I divide by 25, I have remainder a Sandy: If I divide by 25, I have remainder b Peter and Sandy (in unison): Got it!