Overheard: How many people left? (early addition and subtraction)

Theo: you guys go to the library lah, I will sleep by myself Mum: then who will take care of you? Theo: zsa zsa lah Mum : if both of us go, how many people will be left in the house? Theo (without battling an eyelid) : Three Mum (surprised): Huh? How many? Theo: Three Mum (not convinced) :who are they? Theo: Me, Zsa Zsa and Elias. Dad: if ah ma comes over, then how many people in the house? Theo: Four lah