alanforde800Maximus wrote:

A certain sequence consist of alternating positive and negative numbers. If the sequence begins with a negative number and contains K numbers, where K is odd, how many positive numbers are in the set?

a) (k+1)/2

b) (k-1)/2

c) k/(2+1)

d) k/(2-1)

e) k/2

Another approach it to

look for a pattern...k = 1

NEGATIVE

0 positive numbers

k = 3

NEGATIVE, POSITIVE, NEGATIVE

1 positive number

k = 5

NEG, POS, NEG, POS, NEG

2 positive numbers

Notice that if we examine the

first 4 numbers, HALF are positive (and the last number is negative).

k = 7

NEG, POS, NEG, POS, NEG, POS, NEG

3 positive numbers

Notice that if we examine the

first 6 numbers, HALF are positive (and the last number is negative).

k = 9

NEG, POS, NEG, POS, NEG, POS, NEG, POS, NEG

4 positive numbers

Notice that if we examine the

first 8 numbers, HALF are positive (and the last number is negative).

In general, if we examine the first k-1 numbers, HALF will be positive (and the last number is negative).

So, the number of positive numbers = HALF of (k-1)

= (k-1)/2

Answer:

_________________

Brent Hanneson – Founder of gmatprepnow.com