A sample of poly(vinyl chloride), repeat unit $-[\mathrm{CH}{}_2-\mathrm{CHCl}]{}^{-}$ (C = 12, H = 1, Cl = 35.5), consists of 120 chains of molar mass $15000\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$, 200 chains of $35000\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ and 80 chains of $88000\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. Use $M_n=\sum N_i{M}_i/\sum N_i$ and ignore end groups. Calculate the number-average degree of polymerisation, to the nearest integer.

Numerical answer type, tolerance 0.5.