Simple Question

During a Mathematics class, the teacher asks, "Class, who knows what 2 + 2 is?"

Realising how simple the question, Johnny raises his hand, but the teacher pointed to another student. Johnny was upset.

The teacher asks again, "Who knows what 5 + 5 is?"

Johnny raises his hand again, but the teacher still didn't point at him.

The teacher asks his third question, "Johnny, What's X if 40X + 30 = 120X?"

Johnny replies, "But sir, I did not raise up my hand."