Why did Mohammed Chacha refuse the twenty rupees?

Every boy in the colony took his bicycle to Mohammed Chacha, who sat under the peepal tree near the bus stop with a tin box of tools and a bucket of water. He mended punctures for twenty rupees and never seemed to hurry.

When Arnav's tyre went flat on the morning of his maths exam, he ran to the tree. "Please, Chacha, quickly," he said. "I'm late."

Mohammed Chacha looked at him over his glasses. Then he did something strange. Instead of taking the tyre, he handed Arnav the tools.

"You do it," he said. "I will tell you how."

Arnav stared. "But I'll be late!"

"You will be faster next time," said the old man calmly.

So, with Mohammed Chacha's voice guiding him, Arnav took off the wheel, pulled out the tube and pushed it under the water in the bucket until a line of tiny bubbles showed where the hole was. He roughened the rubber, spread the glue, pressed on the patch and counted to sixty. His hands were black and his heart was racing, but the patch held.

He was ten minutes late for the exam, and the teacher frowned. But he had done well, and that evening he went back to the peepal tree with twenty rupees.

Mohammed Chacha pushed the money away. "You did the work," he said. "Keep it. Buy a puncture kit."

Arnav did. The next time his tyre went flat, he mended it himself in eight minutes, and he was not late at all.