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Some water becomes contaminated with radioactive iodine-131 (
). The activity of the iodine-131 in 
of this water is 
. The half-life of iodine-131 is 8.1 days.
Define radioactive half-life.
Calculate the number of iodine-131 atoms in of this water.
An amount of 
of water has a mass of 
. Calculate the ratio
An acceptable limit for the activity of iodine-131 in water has been set as 
. Calculate the time, in days, for the activity of the contaminated water to be reduced to this acceptable level.
.
.
