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, Magoosh Tutor

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

Jan 2, 2016 • Reply

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Official Guide for GMAT Review 2016

Official Guide for the GMAT 2015 14th Ed.

Official Guide for the GMAT 13th Ed.

Official Guide for the GMAT 12th Ed.

Revised GRE PDF 2nd Ed.


Section 5.3 Problem Solving

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