C = C0 e-lefft, or ln C/C0 = -lefft

ln (560/1000) = -leff(2.0 days)

leff = 0.290 d-1

Teff = ln2/0.290 d = 2.39 d

1/2.39 = 1/8.1 + 1/Tb

Tb = 3.4 d

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