That Sum of Which is / That Son of Bitch is

A little boy was doing his Maths homework.

He said to himself, "Two plus five, that son of bitch is seven.
Three plus six, that son of bitch is nine . . ."

His mother heard what he was saying and gasped, 'What are you doing?'

The little boy answered, 'I'm doing my Maths homework, Mum.'

"This is how your teacher taught you to do it?' the mother asked.

"Yes," the boy answered.

With full infuriation, the mother asked the teacher loudly the next day, 'What have
you been teaching my son all this while?"  

"How dare you do it?" added the agonised mother.

"Right now, we are learning addition," calmly the teacher replied.

"And are you teaching them to say two plus two, that son of bitch is four?" shouted the inflamed mother.

The teacher erupted into laughters.

After stopped laughing, the teacher said, "What I taught them was, two plus two, THE SUM OF WHICH, is four."

"I'm very sorry for not finding out the truth first teacher," contritely apologised the mother. In a sheepish manner, she quickly walked away.