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Last updated at Jan. 3, 2019 by Teachoo

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Ex 3.7, 6 (Introduction) The traffic lights at three different road crossings change after every 48 seconds, 72 seconds and 108 seconds respectively. If they change simultaneously at 7 a.m., at what time will they change simultaneously again?Let the traffic lights change in 1 second, 2 second, 3 seconds At 6 seconds, all of them change again. Now, this 6 is actually the LCM of 1, 2, 3 So, Time when they change again = LCM of 1, 2, 3 Ex 3.7, 6 The traffic lights at three different road crossings change after every 48 seconds, 72 seconds and 108 seconds respectively. If they change simultaneously at 7 a.m., at what time will they change simultaneously again?Traffic Lights change every 48 seconds, 72 seconds, 108 seconds We need to find the time when they change simultaneously Now, time when they change again will be more than 48, 72, 108 So we find LCM Time when they change again = LCM of 48, 72, 108 LCM of 48, 72, 108 LCM = 2 2 2 2 3 3 3 = 16 27 = 432 Time when they change again = 432 seconds But We need to find time after 7 am So, first we convert 432 seconds into minutes Time = 432 seconds = 432/60 minutes Time = 7 Minutes 12 Seconds Thus Required Time = 7am + 7 minutes 12 Seconds = 7:07:12 am

Statement questions on LCM

How to check if LCM is to be found or HCF

What is the smallest number when divided by numbers 2, 4, 6 gives remainder 0?

What is the smallest number when divided by numbers 2, 4, 6 gives remainder 7?

Example 14 Important

Ex 3.7, 8

Ex 3.7, 4

Ex 3.7, 9 Important

Ex 3.7, 5

Example 13

What will be the smallest 3-digit number which divides 2, 4, 6 and leaves remainder 0?

Ex 3.7, 2

Ex 3.7, 6 Important You are here

What will be the greatest 3-digit number which divides 2, 4, 6 and leaves remainder 0?

Chapter 3 Class 6 Playing with Numbers

Concept wise

- Factors and Multiples
- Perfect number
- Prime and Composite Numbers
- Sum of prime numbers
- Prime numbers from 1 to 100
- Divisibility Tests - Divisibility by 5, 10, 2
- Divisibility Tests - Divisibility by 4, 8
- Divisibility Tests - Divisibility by 3, 6, 9
- Divisibility Tests - Divisibility by 11
- Divisibility Tests - All
- Common Multiples
- Common Factors
- More Divisibility Rules
- Factor Tree
- Prime Factorisation
- Highest Common Factor
- Lowest Common Multiple
- Statement questions on LCM
- Statement questions on HCF

About the Author

Davneet Singh

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.