Question

How many successive odd numbers beginning with 5 amount to 480?

Answer 1

7
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Answer 2

5+7+9+11+. . . .+n = 480

Sn = [2a+(n-1)d]

a = 5

n = ?

d = 2

[2a+(n-1)d] = 480

n[2(5)+(n-1)2] = 960

n[10+(n-1)2] = 960

10n + 2n2 -2n = 960

2n2 + 8n -960 = 0

(2n-40) (n+24) = 0

n =  = 20 ; n = -24

Therefore, n = 20