Let the elapsed time be n years
For it renewed after every half year it will be 2n
A=8500
P=850
i=21%
A=p(1+i)2n
8500=850(1+21%)2n
7508500=(1+10021)2n
Taking log on both sides,
log(8508500)=log(1+10021)2n
log(8508500)=2nlog(1+10021)
log(8508500)=2nlog(100121)
1=2n(0.08279)
n=2×0.082791
n=6.0393
n=6 years approximately
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