• NEW! FREE Beat The GMAT Quizzes
    NEW! FREE Beat The GMAT Quizzes
    NEW! FREE Beat The GMAT Quizzes
    Hundreds of Questions Highly Detailed Reporting Expert Explanations TAKE A FREE GMAT QUIZ
  • 7 CATs FREE!
    If you earn 100 Forum Points

    Engage in the Beat The GMAT forums to earn
    100 points for $49 worth of Veritas practice GMATs FREE

    Veritas Prep
    VERITAS PRACTICE GMAT EXAMS
    Earn 10 Points Per Post
    Earn 10 Points Per Thanks
    Earn 10 Points Per Upvote
    REDEEM NOW

Divisibility/factors

This topic has 1 expert reply and 0 member replies

Divisibility/factors

Post
How many multiples of 5 are there between 81 and 358?
A. 54
B. 55
C. 56
D. 57
E. 58
i tried this formula $$sum\ =\ \frac{\left(a1+an\right)}{2}\cdot n$$
OA is b

what approach can i use to reach my destination happily? Thanks

  • +1 Upvote Post
  • Quote
  • Flag

GMAT/MBA Expert

Post
Quote:
How many multiples of 5 are there between 81 and 358?
A. 54
B. 55
C. 56
D. 57
E. 58
i tried this formula sum = (a1+an)2⋅n
sum = (a1+an)2⋅n

OA is b

what approach can i use to reach my destination happily? Thanks
Hi Roland2rule,
Let's take a look at your question.
It's a bit tricky one.

We are asked to find the number of multiples of 5 between 81 and 358.
We know that first multiple of 5 between 81 and 358 will be 85 and the last one would be 355.
So why not write an arithmetic sequence of multiples of 5 starting from 85 for this, like:
$$85,\ 90,\ 95,\ ..,\ 355$$

Since this is a sequence of multiples of 5, so number of terms in this sequence will give us the number of multiples of 5 between 81 and 358.
We know that in an arithmetic sequence,
$$a_n=a_1+\left(n-1\right)d ... (i)$$

In the sequence given above,
$$a_1=85,\ a_n=355,\ d=5$$

Plugging in these values in eq (i),
$$355=85+\left(n-1\right)5$$
$$355-85=\left(n-1\right)5$$
$$270=\left(n-1\right)5$$
$$n-1=\frac{270}{5}$$
$$n-1=54$$
$$n=54+1$$
$$n=55$$

It means that the number of terms in this sequence is 55.
Therefore, the number of multiples between 81 and 358 is 55.

Option B is correct.

Hope it helps.
I am available if you'd like any follow up.

_________________
GMAT Prep From The Economist
We offer 70+ point score improvement money back guarantee.
Our average student improves 98 points.

  • +1 Upvote Post
  • Quote
  • Flag
Free 7-Day Test Prep with Economist GMAT Tutor - Receive free access to the top-rated GMAT prep course including a 1-on-1 strategy session, 2 full-length tests, and 5 ask-a-tutor messages. Get started now.
  • Varsity Tutors
    Award-winning private GMAT tutoring
    Register now and save up to $200

    Available with Beat the GMAT members only code

    MORE DETAILS
    Varsity Tutors
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep
  • The Princeton Review
    FREE GMAT Exam
    Know how you'd score today for $0

    Available with Beat the GMAT members only code

    MORE DETAILS
    The Princeton Review
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep
  • PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep
  • e-gmat Exclusive Offer
    Get 300+ Practice Questions
    25 Video lessons and 6 Webinars for FREE

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider

Top First Responders*

1 Ian Stewart 44 first replies
2 Brent@GMATPrepNow 34 first replies
3 Scott@TargetTestPrep 32 first replies
4 Jay@ManhattanReview 31 first replies
5 GMATGuruNY 18 first replies
* Only counts replies to topics started in last 30 days
See More Top Beat The GMAT Members

Most Active Experts

1 image description Scott@TargetTestPrep

Target Test Prep

139 posts
2 image description Max@Math Revolution

Math Revolution

90 posts
3 image description Ian Stewart

GMATiX Teacher

53 posts
4 image description Brent@GMATPrepNow

GMAT Prep Now Teacher

51 posts
5 image description Jay@ManhattanReview

Manhattan Review

31 posts
See More Top Beat The GMAT Experts