Let ${\mathrm{x}}_1,{\mathrm{x}}_2,{\mathrm{x}}_3,\dots \dots {\mathrm{x}}_{\mathrm{n}}$ be n Observations ${\mathrm{w}}_{\mathrm{i}}={\mathrm{lx}}_{\mathrm{i}}+\mathrm{k}$  for $\mathrm{i}=1,2,3\dots \dots \mathrm{n}$where l and k are constants. If the mean of 'x_i s is 48 and their S.D is 12 the mean of ' w_i s is 55 and S.D of '  w_i s is 15 then the values of l and k should be