The human serum, albumin can be labelled with atoms of the radioactive isotope iodine 131. This has a physical half life of 8 days and a biological half-life of 21 days. Find the effective half-life.

1/T_E = 1/T_P + 1/T_B

1/T_E = 1/8 + 1/21

T_E = 6 days (1 significant figure answer required)