It will take nearly 14 years for the balance to be double .
In compound interest, the amount is given as below,
[tex]A=P(1+\frac{r}{100} )^{n}[/tex]
where r is rate of interest and n is number of years.
Given that, [tex]A=800,P=400,r=5[/tex]
Substitute values in above formula.
[tex]800=400(1+\frac{5}{100} )^{n} \\\\(\frac{21}{20} )^{n} =2\\\\n=14.2 years[/tex]
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