Half-life = 5 years. After 20 years there have been ( \dfrac{20}{5} = 4 ) half-lives.

[ 68 = A_0 \left(\frac12\right)^4 = A_0 \cdot \frac{1}{16} \implies A_0 = 68 \times 16 = 1088. ]

Answer: 1088 microcuries