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Source: Official Guide for GMAT Review 2016 Problem Solving; #39

2

In a certain sequence of 8 numbers

In a certain sequence of 8 numbers, each number after the first is 1 more than the previous number. If the first number is ? 5, how many of the numbers in the sequence are positive?

1 Explanation

2

Jeffrey Braganza

Use this explicit formula for sequence of numbers:

an = a1 + (n-1)*d
an = number of the nth term
a1 = number of first term
n = total no of terms in the sequence
d = difference between each term

Using the above formula, we will find the no. of terms 'n' from -5 to 0
Given: Total no of terms = 8
So, numbers of terms positive = (Total no of terms) - (no of terms 'n' from -5 to 0)

an = 0
a1 = -5
n = ??
d = 1

0 = -5 + (n-1)*1
n = 6

No of terms positive = 8 - 6 = 2
(C)

Dec 31, 2015 • Comment

Sam Kinsman

Yes, this method works well. Good job! :)

Jan 2, 2016 • Reply

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