The half-life of fluoxetine and its active metabolite norfluoxetine is respectively 2 to 4 days and 7 to 15 days

N = initial_concentration*2^(-t/half_life)

N = (20/77)*2^(-14/3)

N = 0.0102

0.0102 = (20/77) * 2^(x/0.5416)

solve for x?