What was Mr Iyer's mistake?

Mr Iyer wrote the sum on the board, worked it out, and drew a box around the answer: 48.

Priya looked at her notebook. She had got 42. She worked it again. Still 42. She looked at the board and followed each line, and there it was: in the third line, Mr Iyer had written 7 × 6 as 48.

Her hand went halfway up and stopped. Mr Iyer had taught for twenty years. Nobody corrected Mr Iyer.

She put her hand down. Then she thought of Rahul in the back row, who copied everything from the board and never checked, and who would learn the wrong answer. Her hand went up again, all the way this time.

"Sir," she said, "I think in the third line, seven sixes might be forty-two."

The class went silent. Mr Iyer turned to the board and looked at it for a long moment. Then he picked up the duster and rubbed out the 48.

"Forty-two," he said. He wrote it in. "Thank you, Priya." And then, to the class, "Checking is not rude. Not checking is."

After that, whenever he finished a sum on the board, he would turn around and ask, "Well? Who is going to check me?"