$$9 - 1 = 10 = 11 - 1,$$ thus
$$29 - 1 = 20 + 9 - 1 =\\ = 20 + 11 - 1 = 31 - 1 = 30.$$
Edit
Or just using that $9-1 = 10$
$$ 29 - 1 = 20 + 9-1 = 20 + 10 = 30.$$
Olba12
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