<p>One litre of hard water contains 12.0 mg&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>M</mi><msup><mi>g</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></math>&nbsp;ions. Milli equivalent of washing soda required to remove its hardness is:</p>