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.