The half life of radioactive radon is 3.8 days . The time , at end of which $\;\large\frac{1}{20}^{th}\;$ of radon sample will remain undecayed is

Question

$(a)\;18 \;days\qquad(b)\;16.5 \;days\qquad(c)\;17.6 \;days\qquad(d)\;19.2 \;days$