Find m if the sum of the consecutive integers from -16 to m, inclusive

Question

Find m if the sum of the consecutive integers from -16 to m, inclusive is 74.

A) 17
B) 18 
C) 19 
D) 20
E) 21

BeingHan · Accepted Answer

It will be D 20
As -16 to 16 will cancel out the addition. And then you can simply add 17 ,18 ,19 and 20 to get 74.

Sent from my Moto G (5) Plus using  GMAT Club Forum mobile app

GMATinsight · Answer

Sum of all consecutive integers from -16 to +16 = 0

Now sum of 17+18+19+20=74

Hence m= 20

Answer option D

Bunuel  please increase one option here to make it a GMAT like question .

Add an option E)21

Bunuel · Answer

Done. Thank you for the suggestion.

arvind910619 · Answer

Imo D 
Can be calculated from sum formula of a sequence

Sent from my ONE E1003 using  GMAT Club Forum mobile app

umadurga · Answer

Opt D
Other way round
S = n/2(2a+(n-1)d)
=> 74=n/2(-32+n-1) as common difference is 1
148=-33n+n^2
n^2-33n-148=0
upon simplification n = 37
tn=a+(n-1)d
m=16+(37-1)1
m=20

JeffTargetTestPrep · Answer

We can use the following formula:
 
sum = average x quantity
 
Since we have a set of evenly spaced integers, the average is (-16 + m)/2.
 
We can also calculate the quantity as m - (-16) + 1 = m + 17.
 
Thus:
 
74 = (-16 + m)/2 x (m + 17)
 
148 = (m - 16)(m + 17)
 
148 = m^2 + m - 272
 
m^2 + m - 420 = 0
 
(m + 21)(m - 20) = 0
 
m = -21 or m = 20
 
Since m > -16, m must be 20. 
 
Alternate Solution:
 
The integers in the sum will be -16, -15, -14, …, -2, -1, 0, +1, +2, …, m. Notice that all the negative integers from -16 to -1 inclusive will be offset by the positive integers from +1 to +16, yielding a net sum of 0. So now, starting with the integer +17, we need to add consecutive integers until we get a sum of 74. We see that 17 + 18 + 19 + 20 = 74, and thus m = 20, the largest integer in the set. 
 
Answer: D

gracie · Answer

(17+m)=74→
m^2+m-420=0→
x=20
D

liusam · Answer

That's take me too long about 10 mins to prove it's D. I need more practice.

MathRevolution · Answer

First term: -16 and Last term : m

Sum: 74

Sum from -16 to +16 will be '0', hence to get 74 as the sum, we will have to extend the series ahead.

17 + 18 + 19 + 20 = 74.

Therefore, m = 20

Answer D

ManifestDreamMBA · Answer

All terms till 16 will cancel out.

Now we have 17 + 18 +19 ...n terms = 74
=17 +17 +1 +17+2....n terms
=17n +n(n-1)/2 = 74 (note we have n-1 terms 1,2,3...)
34n +n^2 -n = 148
(n+34)(n-4) =0
n = 4
there are 4 terms (including 17)
17+3 = 20, Answer

Find m if the sum of the consecutive integers from -16 to m, inclusive

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

Find m if the sum of the consecutive integers from -16 to m, inclusive

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards