Question

Find the sum of all two digit natural numbers which are divisible by 5

Answer 1

Heya

The two digit natural numbers divisible by 5 are

10,20......90,95

The series forms an A.P

an = a + (n-1)d

95 = 10 + (n-1) 5

95-10 = (n-1)5

85/5 = n-1

17 = n-1

17+1 = 18 = n

Sum of n numbers in a series is given by,

Sn = n/2(a+an)

= 18/2 (10+95)

= 9×105

= 945

Sum of all two digit numbers divisible by 5 is 945

Hope it helps!