How to get 15% here (the right answer)?

ZnCl

_{2} Zn

^{2+}+2Cl

^{-}I calculated the degree: n(ZnCl

_{2} that dissociated)/n(ZnCl

_{2} at the beginning)*100 and got 13%.

n(ZnCl

_{2} that dissociated) is equal to n(Cl

^{-})/2.

n(ZnCl

_{2} at the beginning) is equal to n(ZnCl

_{2} that dissociated) + n of the undissociated salt.

It can't be a mistake in calculation.