Let ψa(x) and ψb(x) be two orthonormal solutions of the time independent Schr\"odinger equation for a given potential with energy eigenvalues Ea and Eb. At the time t=0, suppose that the system is in the state
ψ(x,t=0)=21(ψa(x)+ψb(x)).
Discuss the probability density at a later time t.