Two pipes A and B can fill a tank in 21 hours and 18 hours, respectively. If both the pipes are opened simultaneously, then the time taken to fill the tank is:
1
\(11 \frac{27}{39}\) hours
2
\(10 \frac{27}{39}\) hours
3
\(8 \frac{27}{39}\) hours
4
\(9 \frac{27}{39}\) hours
5
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