HCF LCM doubt

Question

How to determine whether a question is of LCM or HCF ?? It's sometime very confusing. Please help. Thank-you in advance.

Comments

Plz ask ur question in proper way, in the sense what u want to ask????????

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 The policemen at three different places on a ground blow a whistle after every 42 sec, 60 sec and 78 sec respectively. If they all blow the whistle simultaneously at 9:30:00 hours, then at what time do they whistle again together ? 

In these type of questions, how do we know , which one to apply HCF or LCM ???

Answer 1

It is an question of LCM and rather it is also an question of arithmetic series also

First we write factors of each number

42=2*3*7

60=2*2*3*5

78=2*3*13

LCM (42,60,78)=LCM($2^{1}*3^{1}*7^{1}$;$2^{2}*3^{1}*5^{1}$;$2^{2}*3^{1}*13^{1}$)=2²*3¹*5¹*7¹*13¹=5460 sec

5460 sec=91 min=91 min+9:30:00= 11.01.00 hours

so at 11.01.00 they should whistle again