GMAT Ninja Quant Ep 10: Counting & Sequences

Do you get intimidated when you're asked to find the sum of a large set of numbers? Are you bamboozled by subscript notation or confused by sequences questions?

In this video, Harry -- a GMAT Ninja tutor -- will help you understand what to look for in a counting or a sequences question. He'll help you understand some of the patterns buried in these questions so that you don't have to rely on memorizing a formula.

This video is one of the easier videos overall, but there are still a few questions that might challenge you, even if you’re currently scoring well into the 40s.

This is video #10 in our series of full-length GMAT quant lessons. For updates on upcoming videos, please subscribe!

This video will cover:
➡️ Pattern recognition
➡️ Finding the number of terms in a list
➡️ Finding the sum of a list of numbers
➡️ Understanding subscript notation
➡️ Converting sequences questions into algebra

This video is for you if:
➡️ You get intimidated by big numbers or weird notation
➡️ You’re performing a bunch of steps but don’t know why
➡️ You don’t feel like you intuitively understand what you need to do

Want more GMAT and EA test-prep tips and advice?

Chapters:
00:00 Introduction
03:18 Question 1 - The Fencepost Problem
07:56 Question 2 - Summing a Sequence
17:07 Question 3 - The Mean Matches the Median
23:19 Question 4 - Subscript Notation
32:20 Question 5 - Exponential Growth
39:00 Question 6 - Where does the sequence start?
43:41 Question 7 - A Repeating Sequence
49:38 Question 8 - Division in the Formula
55:25 Question 9 - The Product of a Sequence

[abhishekde]

Harry is the best teacher out there..period.. so polite and loves his job

[UnibrowSaggin]

Huge thanks Harry for your help (+ the rest of the team), i'm following the team's videos for 2 weeks now, it's life changing !

[apoorvsinha]

Your method for solving the Q3 was really AMAZING. In my attempt, i had to use the formula of the SUM of AP progression and then i used the nth term formula to get to the answer. CHEERS...!!

[shubhshinde]

The most challenging aspect of the GMAT is the language of math, rather than the math itself.

[akshitsingh]

That's really a quality content. Everything explained so well. Thanks Harry

[deansakata]

2nd to last Q is a well constructed puzzle! love it!

[ryanshotbolt]

Great explanations of everything. Thanks!

[hardikarora]

Found a shortcut manner to solve Q2 (bank balance one) which uses the GMAT's tricks against the exam itself.

Since I know that a lot of people will forget to add the initial $100, I know that GMAT will include a trap answer which is $100 less than the actual answer. As a result, by looking at the answer choices we can ascertain that $4650 and $4750 are the only options with a difference of $100. Thus, the trap answer is $4650 (B) and the actual answer is $4750 (C)

This isn't a sure shot way to solve it but if you are low on time then can give you a good manner to catch up while being reasonably certain.

[SaloniGoel]

Thank you for the quality content! W.r.t the guinea pigs population, when they say pigs added to the population in the next 6 months, this is my thought process -

Starting with 10 guinea pigs today and the number doubling each month:

Month 1: 10 guinea pigs
Month 2: 20 guinea pigs
Month 3: 40 guinea pigs
Month 4: 80 guinea pigs
Month 5: 160 guinea pigs
Month 6: 320 guinea pigs

For the next 5 months (months 2 to 6), you would be adding:
20 + 40 + 80 + 160 + 320 = 620 guinea pigs. What is going wrong here?

I used the formula for sum of GP and got a6 = 630 which implies that 630-10=620 guinea pigs were added as well

[frederickkingjnr]

for Q5, i used the compound interest formula and calculated 10(1 + 1)^6 = 10(2)^6 and got my answer quicker

[KunalSethi]

For Q4, I back-tracked a7=13 to a1 = -5 and then I added 3 (41 times) to reach 123-5 = 118

[rodamaster]

I got Q3 right but it took me 3mins. That property is great, thanks so much for the video!

[hikatube]

This was a great video.

For Question 6: the "must" part, can the same logic be reversely used? Meaning, just because 4 is in the set, P=1 then 1 & 4 is in the set but not necessary 7 & 13 "MUST" be in the set. 4 is not defined as P so I don't see a logic where 7 and 13 "must be in" We can have cases like just 7 was in the set (and set ends at 7), 1 and 4 only in the set, 1, 4, and 7 are the only set, ..

[mandilwar]

For q1, cant we say 1st tree will be on 0th metre, so total=30+1???

[shivamsinghal]

In first question, it is clearly written that plant a tree every 15 meters

[Sergio-knxw]

a better way its using the trick that 120+180 = 300. 122+178 = 300.... always 300 then u know there are 15 pairs + the number 150 which does not have a pair due to that theare 31 numbers between 120 and 180. So the answer is 100 + 15 x 300 + 150 = 4750. 25 seconds

[kelvincheng]

On Q8 for the 1st statement (and on similar questions) how do we make sure to read as in your explanation a8 = a6/a7 (all together in one equation) and not confuse it for something like the product of a7 and a8 as a7 = a5/a6 times a8 = a6/a7 (the whole sequence for each)?
Is there a rule or is just some intuition after doing several exercises?

BTW, Happy New Year and thank you so much for all the help you have given us.

[HaraChiaki]

Hi, is it possible to have the answers for the 2 examples at the very beginning, please? Thanks! :)

For the first one I got B 27 720$
Second B 34. but really not sure (':

[ayanmodi]

In question 6 - How can we assume that even 13 will be in the set, what if the set stops at 4 and 7? that information is also not stated, PS - it is a brilliant ssession - Thank you so much GMAT Ninja!

[corrayatom]

The technique showed in 16:39 really works 💜 Here's some cases :

i) 2+4+6+8+10 (5 terms)

so, 10-2 = 8
8÷2 = 4
4+1 = 5 (5 terms)

BUT,
ii) 3+6+9+12+15+18 (6 terms)

so, 18-3 = 16
16÷3 = 5.something
5.something + 1 = 6.something

WHAT TO DO NOW? So, is it like if there a scenario is like this we will consider 6.something as 6 (6 terms)

Right?