# It's a ionizzzzzer!

Long-acting ionizer that create per unit time per unit volume of air ion pairs $$n_i$$ = $$3.5 \times 10^{9} cm^ {-3} s^{ -1}$$ , has been turned off. Assuming that the only process tending to reduce the number of ions in the air is their recombination coefficient r = $$1.67 \times 10 ^{-6} cm^{3} /s$$, find how long after turning off the ionizer, ion concentration decreased $$2.0$$ times.

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