A ratio active substance dis integrals at a rate proportional to amount of substance present. $50 \%$ of amount disintegrates in $1000$ years. Approximately what $\%$ of substance will disintegrate in $50$ years ?

Question

$(a)\;(1-(\frac{1}{2}) ^{1/20}) \times 100 \\ (b)\;(1-(\frac{3}{4}) ^{1/20}) \times 100 \\ (c)\;(\frac{1}{2}) ^{1/20} \times 100 \\ (d)\;(1-(\frac{3}{2}) ^{1/20}) \times 100$