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Neptunium-239 has a half-life of 2.35 days. How many days must elapse for a sample of 239-Np to decay to 1.00% of its original quantity?

I got: close to 15 days. Is this right?
I figured that about 1% of the original quantity would be roughly 6.5 Half lives, and since a half life is 2.35 days, 6.58*2.35

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  1. It's done this way for an exact answer.
    k = 0.693/t1/2. Substitute k into the following equation and solve for t.
    ln(100/1) = kt.
    I get 15.6 days.

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