Find the sum of first 20 natural numbers which are divisible by7.​

2 thoughts on “<br />Find the sum of first 20 natural numbers which are divisible by7.​”

1. Answer:

   • 1470.

   Step-by-step explanation:

   • The number series 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, . . . . , 140.
   • Therefore, 1470 is the sum of first 20 positive integers which are divisible by 7.

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2. Answer:

   The average of first 20 natural numbers which are divisible by 7 is 73.5.

   Step-by-step explanation:

   To find : The average of first 20 natural numbers which are divisible by 7 ?

   Solution :

   First we find the sum of first 20 natural numbers which are divisible by 7.

   i.e. 7,14,21,…. forming an A.P.

   Where, first term is a=7

   Common difference is d=7

   Number of terms n= 20

   The sum of n terms of A.P is S_n=\frac{n}{2}[2a+(n-1)d]S

   n

   =

   2

   n

   [2a+(n−1)d]

   S_{20}=\frac{20}{2}[2(7)+(20-1)7]S

   20

   =

   2

   20

   [2(7)+(20−1)7]

   S_{20}=10[14+133]S

   20

   =10[14+133]

   S_{20}=10[147]S

   20

   =10[147]

   S_{20}=1470S

   20

   =1470

   Therefore, the average of first 20 natural numbers which are divisible by 7 is 73.5.

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