Answer:

495

Step-by-step explanation:

20 + 25 + 30 + 35 + . . . . . + 70

This is an arithmetic progression

The initial term of this arithmetic progression is a1 = 20

The n-th term this arithmetic progression an = 70

The common difference of successive members is d = 5

Use formula for n-th term of the sequence :

an = a1 + ( n - 1 ) * d

In this case :

a1 = 20

an = 70

d = 5

n = ?

an = a1 + ( n - 1 ) * d

70 = 20 + (n - 1) * 5

70 - 20 = 5n -5

50 = 5n - 5

50 + 5 = 5n

55 = 5n

55 = n

5

11 = n

n = 11

The sum of the members of a arithmetic progression is :

Sn = ( n / 2 ) * ( a1 + an )

    = 11 / 2 * ( 20 + 70 )

    = 11 / 2 * ( 90 )

    = 990 / 2

Sn  = 495

Step-by-step explanation:

hope u understand