A number is doubled and 9 is added

A number is doubled and 9 is added. If the resultant is trebled, it becomes 51. What is that number?

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 #10331

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