By selling 99 pens, a trader gains the cost of 9 pens

By selling 99 pens, a trader gains the cost of 9 pens. Find his gain percentage?

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

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AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #61102

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