Help please. Thank you very much?

Molar mass of Ca²⁺ = 40.1 g/mol = 40.1 mg/mmol
Concentration of Ca²⁺ = (40 mg/L) / (40.1 mg/mmol) = 40/40.1 mmol/L

Molar mass of Mg²⁺ = 24.3 g/mol = 24.3 mg/mmol
Concentration of Mg²⁺ = (10 mg/L) / (24.3 mg/mmol) = 10/24.3 mmol/L

Hardness in mmol/L = [(40/40.1) + (10/24.3)] mmol/L

Molar mass of CaCO₃ = (40.1 + 12.0 + 16.0×3) g/mol = 100.1 mg/mmol
Hardness = {[(40/40.1) + (10/24.3)] mmol/L} × (100.1 mg/mmol) = 141 mg/L