A consolidated$-$drained $($CD$)$ triaxial test is conducted on a specimen of sandy soil, which the cohesion $($c$\text{'})$ can be assumed to be $0.$ The total stress confining pressure $($i$.$e$.$ cell pressure$)$ is $230$ kPa, and back pressure is $90$ kPa. At failure, the applied vertical force is $449$ N and the cross$-$sectional area is $1698$ mm$2.$ Estimate the friction angle of the sand in degree.