Divide by the molar mass of calcium (found on the periodic table).
Rigorously:
425g x 1 mol / 40.078g = 10.060432157 moles
Rounded down to proper significant figures: 10.6g
Rigorously:
425g x 1 mol / 40.078g = 10.060432157 moles
Rounded down to proper significant figures: 10.6g
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425/40 = 10 5/8 moles.