(2R,3R)-2,3-Dimethyloxirane reacts with $\mathrm{NaN}{}_3$ in aqueous solution (followed by protonation). Azide attacks one ring carbon by SN2. The product is

Attack at either carbon gives the same compound because the epoxide has $C_2$ symmetry. The carbon attacked by azide is inverted, and its priority order (N or O $>$ the other stereocentre $>$ $\mathrm{CH}{}_3$ $>$ H) is unchanged, so its label changes from R to S. The carbon that keeps the oxygen is untouched and stays R. This gives the C-2 (OH) = R, C-3 (N3) = S product.