The formula for standard error is $\sqrt{\frac{p(1-p)}{n}}$, where p is the population proportion and n is the sample size. Here, p is $20 \%$ or $0.20$ and n is $50$. So, the standard error is $\sqrt{\frac{0.20 \times 0.80}{50}}=0.057$ or $5.7 \%$.
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