Find decay constant: N/N_{0} = 1/3.5 = e^{-k6} ln (1/3.5) = -k6 k = 0.209 d^{-1}

The number declines from 1552 on day 3 to 40% lower than 852, i.e., 60% of 852 = 511.

511 = 1552e^{-0.209t} ln 511 = 1552 - 0.0209t t = 5.32 days after day 3 i.e., 8.32 days after the present.

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