Question

find the sum of all multiples of 5 lying between 101 and 999

Answer 1

a = 105

an=995

d =5

an= a + {n-1}d

995 = 105 +{n-1}5

995 -105 = 5n -5

 890 = 5n -5

 890 + 5 = 5n

 n = 895\\5

n = 179

sn = n\\2 { a + l}

sn = 179\\2{105 +995}

sn = 179\\2{1100}

sn =179 {550}

 sn =98450

Answer 2

First multiple 105
last multiple 995
No of terms=?
now An= a+ (n-1)d
995=105+(n-1)5
(n-1)=178
n= 179
sum= 179/2(105+995)
sum= 98450