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Guided, difficulty 1 of 3

Correcting a known bias

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A water-quality team measured the chlorine level (mg/L) at six taps:

5.19, 4.05, 4.19, 4.29, 4.96, 4.56

Later it found that the meter reads 0.15 mg/L too high on every measurement. What is the corrected mean chlorine level? Give 3 decimal places.