What is the concentration of NaCl, in \({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\), when \({\text{10.0 c}}{{\text{m}}^{\text{3}}}\) of \({\text{0.200 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) NaCl solution is added to \({\text{30.0 c}}{{\text{m}}^{\text{3}}}\) of \({\text{0.600 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) NaCl solution?

A.     0.450

B.     0.300

C.     0.500

D.     0.800

C

One respondent suggested that this question was mathematically difficult. However, 54.42% of candidates did get the correct answer C.