At $3 a dozen, Krista sold $120 / $3 = <<120/3=40>>40 dozen eggs.
If 10 hens laid 40 dozen eggs in 4 weeks, that means 40 dozen eggs / 10 hens = <<40/10=4>>4 dozen eggs were laid by each hen in four weeks.
To find out how many eggs each hen laid in a week, 4 dozen eggs / 4 weeks = <<4/4=1>>1 dozen eggs each week.
Since a dozen has 12 eggs, each hen lays 12*1 = <<12*1=12>>12 eggs each week.
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