How did Meera use "the answers themselves as a clock"?

Halfway through her first board exam, Meera looked at her watch and froze. It said 10:15, exactly what it had said the last time she looked. The battery had died.

There was no clock on the wall of the exam hall, and the invigilator was busy at the far end of the room. Meera felt her heart race. How much time was left? An hour? Twenty minutes? She began to write faster and faster, until her handwriting was hard to read even for herself.

Then she stopped and took a breath. Panicking, she realised, was wasting more time than the broken watch. She had practised this paper at home many times, and she knew that each long answer took her about fifteen minutes. So she would use the answers themselves as a clock. Three long answers were left: about forty-five minutes of work.

She wrote steadily, one answer at a time, without looking at her wrist again. She had just finished the last line of the last answer when the bell rang.

Outside, her friends were complaining that the paper had been too long. Meera only smiled and slipped the dead watch into her pocket. That evening she bought a new battery, but she never forgot the lesson. A watch could stop; what she had practised could not.