Learning curve rate, r, means the ratio of production time satisfies the relation
T_2k=r*T_k
solving the equation gives
T(n)=(T_initial)*(n^b) where b=log(r)/log(2)
Here, r=0.9 => b=log(0.9)/log(2)=-0.15200
n=12, T_initial=20
=>
T(12)=20*12^(-0.15200)=13.71 hours