EX3.7 problem sums (post 3)

x 3.7 part 2

April 20, 2020

Q5. Determine the greatest 3 digit no exactly divisible by 8, 10 and 12 
Greatest 3 digit number = 999

Find LCM of 2|8, 10, 12
                        2|4, 5, 6    
                        3|2, 5, 3    
                        5|2,  5, 1   
                        2|2,  1, 1   
                           1,  1,  1

LCM = 120

120) 9 9 9( 8
        9 6 0 
           3 9

999 - 39 = 960

The greatest 3 digit number exactly by indivisibly 8,10, 12 = 960

Q6. The traffic lights at 3 different road crossing change after every 48 seconds 72 seconds and 108 second respectively. If they change simultaneously at 7:00 am at what time will they change simultaneously again?

[HINT: Simultaneously means together --> Find LCM]

Answer: 2|48, 108, 72 
               2|24, 54, 36   
               2|12, 27, 18   
               3|6, 27,   9     
               3|2,  9,   3      
               2|2,  3,   1      
               3|1,  3,   1      
                  1,  1,   1

LCM = 3 x 3 x 3 x 2 x 2 x 2 x 2
          = 27 x 16
          = 432 sec
Change to minutes 
60 Sec = 1 Min 
                         ______
Therefore    60) 4 3 2 ( 7
                           4 2 0 
                              1 2 sec

                         7 min 12 sec
So the light changes again at (7: 07: 12 am)
                                 (7: 00 + 7 min + 12 sec)


Do the Following exercises in your copy

Q1. find the greatest 4 digit number that is exactly divisible  by 12, 15, 18 and 27?

Q2. Two neon light blink every 20 sec and 30 sec respectively. Given that they flashed together at 8 pm, when will they flash together the next time?


Check your answer below:

Q1. 9720
Q2. 8: 01pm 

Thankyou boys 
have a nice day