Math, asked by darky8535, 1 year ago

The number of numbers lying between 100 and 500 divisible by 7 but not by 21 is? Please give a simple and shortcut method.

Answers

Answered by tnrao74owzfhb
19
Two ways to do it, the long way is to start listing them:

105

112

119

126

133

etc

 

and then finding the first one divisible by 21 (hint, it's 105) and then crossing out every third one.

 

The second way is subtract 500 - 100 = 400, so there are 400 numbers to consider, of which 1/7 is divisible by 7, so

400/7 = 57.1 which means there are 57.  Again, 1/3 of those will be divisible by 21 so 57/3 = 19 that are and 38 that are not.


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