23. Three bells ring at intervals of 15, 20 and 30 minutes respectively. If they all ring at 11.00 a.m. together, at what time will they next ring together? RRB NTPC (Phase 1) 2016 (1) 11.30 a.m. (2) 12 noon (3) 12.30 p.m. (4) 1.00 p.m.

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23. (2) LCM of 15,20 and 30 min, [ begin{array}{c|l} 2 & 15,20,30 hline 2 & 15,10,15 hline 3 & 15,5,15 hline 5 & 5,5,5 hline 1, & 1,1 = & 2 times 2 times 3 times 5 = & 60 mathrm{min} end{array} ] ( therefore quad ) Required time ( =11: 00+1 mathrm{h} ) ( =12 ) noon

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