A thin prism of angle \[15{}^\circ \] made of glass of refractive index \[{{\mu }_{1}}=1.5\] is combined with another prism of glass of refractive index \[{{\mu }_{2}}=1.75\]. The combination of the prism produces dispersion without deviation. The angle of the second prism should be [AIPMT (M) 2011]