Question

find the sum of multiples of 5 lying between 84 and 719.

Answer 1

Smallest multiple of 5 between 84 and 719 = 85

Largest multiple of 5 between 84 and 719 = 715

First Term = a1 = 85

Common Difference = 5

Find the number of terms:

an = a1 + (n - 1)d

715 = 85 + (n - 1)5

715 = 85 + 5n - 5

5n = 635

n = 127

Find the sum:

Sn = n/2 (a1 + an)

Sn = 127/2 (85 + 715)

Sn = 50800

Answer: The sum is 50800

Answer 2

SUM=NO OF TERMS/2(FIRST TERM+LAST TERM)

FIRST TERM=85

LAST TERM= 715

NO. OF TERMS= 4+20+20+20+20+20+20+3

                         =127

SUM= 127(85+715)/2

        = 127*800/2

        =127*400

        =50800