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Arithmetic mean of 4.2 , 2.61 , 3.6:

= $\dfrac{4.2+2.61+3.6}{3}$

= 3.47

Geometric mean of 4.2, 2.61, 3.6

= $\sqrt[3]{4.2*2.61*3.6}$

$\approx 3.405$ (corrected to 3 d.p.)