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If the sum of the consecutive integers from –42 to n inclusive is 372, what is the value of n?

A. 47 B. 48 C. 49 D. 50 E. 51

# of terms =\(42+1+n=(n+43)\)

\(Sum=372=(n+43)*\frac{(n-42)}{2}\)

\(744=(n+43)*(n-42)\)

\(n=50\)

OR

42 terms after zero and 42 terms below zero will total 0. So, our new question will be consecutive integers with first term 43 have sum 372, what is the last term:

If the sum of the consecutive integers from –42 to n inclusive is 372, what is the value of n?

A. 47 B. 48 C. 49 D. 50 E. 51

As suggested above, the quickest way to solve this question would be to ballpark it. We know we are looking for 43 + 44 + 45...... = 372 Now 40*8 = 320.... 372 is more than 320 40*9 = 360.... but it is very close to 372. When we add the numbers, we add 48, 49 etc which have an excess of 8, 9 etc as compared to 40. Hence, we need the sum to be some difference away from 372. Therefore, we need to add only 8 numbers, not 9. Keep in mind, when going from 43 to n to get 8 numbers, n will be 50, not 51. 43 to 50 give us 8 numbers (50 - 43 + 1).
_________________

Nice solution. Correct as always. I was actually thinking of having a ballpark and reach the answer from 43 to 50. Add from 43, 44, 45.. to reach 372, need at least 7 (43x7=301) and less than 9 (43x9=387).

Did you use any intermediary formulas for this part? (n+43)*(n-42)=744 n=50

Sum of consecutive integers or sum of terms in any evenly spaced set (AP): \(\frac{first term+last term}{2}*number of terms\)

In original question: First term: -42 Last term: n Number of terms: 42+n+1=n+43

Nice solution. Correct as always. I was actually thinking of having a ballpark and reach the answer from 43 to 50. Add from 43, 44, 45.. to reach 372, need at least 7 (43x7=301) and less than 9 (43x9=387).

Did you use any intermediary formulas for this part? (n+43)*(n-42)=744 n=50

Sum of consecutive integers or sum of terms in any evenly spaced set (AP): \(\frac{first term+last term}{2}*number of terms\)

In original question: First term: -42 Last term: n Number of terms: 42+n+1=n+43

\(\frac{(n-42)}{2}*(n+43)=372\)

\((n-42)*(n+43)=744\)

Question is how we can solve such complex algebraic equation in short time.

I suggest POE method.

1. Choose option C, put this value in the equation (First selection is based on instinct ) => (49+43)(49-42) = 744 => 92 * 6 = 744 => 553 = 744 Here we can conclude that n should be greater than 49 . Options A,B and C are eliminated

2. Chose option D, put this value in the equation =>(50+43)(50-42) = 744 => 93 * 8 = 744 => 744 = 744 Bingo !!! => n is 50

Thanks guyz for the various ways. OA is D pray the mind in open enough during the actual exam to be able to think out benchmarking and the other techniques!

Here's some more addition from Bunuel.. Look at answer choices:

A. 47 B. 48 C. 49 D. 50 E. 51

n-42 n+43

It can not be A as 47-42=5 and 744 is not a multiple of 5. It can not be B as 48-42=6 and 43+51=91 their multiple will have 6 for the last digit

It can not be C as 49-42=7 and 744 is not a multiple of 7. It can not be E as 51-42=9 and 744 is not a multiple of 9.

Bunel: I understand how you are calculating # of terms given the fact that the sum is +ve. What if the sum were -ve? In that case "n" can be smaller or greater than -42. How do we calculate # of terms in that case. Thank you in advance.

For the number of terms in the set of consecutive integers, it doesn't matter whether the sum is positive or negative.

If we are told that integers from a to b are inclusive then the # of terms would be: last term (biggest)-first term(smallest)+1=b-a+1.

eg. set from -42 to -35 inclusive: -35-(-42)+1=8. OR set from -59 to -42 inclusive: -42-(-59)+1=18 OR set from -1 to 4 inclusive: 4-(-1)+1=6

When a and b are not inclusive, # of terms between a and b: b-a-1
_________________

If the sum of the consecutive integers from –42 to n inclusive is 372, what is the value of n?

A. 47 B. 48 C. 49 D. 50 E. 51

As suggested above, the quickest way to solve this question would be to ballpark it. We know we are looking for 43 + 44 + 45...... = 372 Now 40*8 = 320.... 372 is more than 320 40*9 = 360.... but it is very close to 372. When we add the numbers, we add 48, 49 etc which have an excess of 8, 9 etc as compared to 40. Hence, we need the sum to be some difference away from 372. Therefore, we need to add only 8 numbers, not 9. Keep in mind, when going from 43 to n to get 8 numbers, n will be 50, not 51. 43 to 50 give us 8 numbers (50 - 43 + 1).

We can also use some divisibility properties, because we have a sequence of evenly spaced integers. If the number of terms in the sequence is odd, the sum of the numbers is a multiple of the middle term. If the number of terms is even, the total sum is a multiple of the sum of the two middle terms. If we factorize 372 we get 2*2*3*31. By combining the factors we cannot get a divisor of 372 in the range of 40-50, but 3*31 = 93 = 46+47. So, we have an even number of terms, with 46 and 47 the two middle terms, first term 43, then the last must be 50.

Not shorter than your solution, but it's fun to play with divisibility rules, sometimes just for the sake of practice...
_________________

PhD in Applied Mathematics Love GMAT Quant questions and running.

hello bunuel, i do understand the process. but i really want to know is there any short cut for 744=(n+43)(n-42) to arrive n=50

thanks in advance

n is the number of terms so it must be positive. It must be greater than 42 to ensure that (n - 42) is not negative.

744 needs to be written as a product of two numbers. Factorize 744. You get 744 = 8*93 = 8*3*31 We need one of the numbers to be greater than 85 to account for (n+43) where n is greater than 42. We see 8*93 is possible. If the (n+43) factor is 93 but n must be 50.
_________________

42 terms after zero and 42 terms below zero will total 0. So, our new question will be consecutive integers with first term 43 have sum 372, what is the last term:

\(\frac{43+n}{2}*(n-43+1)=372\)

\((n+43)*(n-42)=744\)

\(n=50\)

Answer: D (50)

Bunuel will you be kind enough to explain this manipulation, I can't seem to get it done! \((n+43)*(n-42)=744\)

Thanking you in advance.

Multiply \(372=(n+43)*\frac{(n-42)}{2}\) by 2 to get \(744=(n+43)*(n-42)\).
_________________

If the sum of the consecutive integers from –42 to n inclusive is 372, what is the value of n?

A. 47 B. 48 C. 49 D. 50 E. 51

# of terms =\(42+1+n=(n+43)\)

\(Sum=372=(n+43)*\frac{(n-42)}{2}\)

\(744=(n+43)*(n-42)\)

\(n=50\)

OR

42 terms after zero and 42 terms below zero will total 0. So, our new question will be consecutive integers with first term 43 have sum 372, what is the last term:

\(\frac{43+n}{2}*(n-43+1)=372\)

\((n+43)*(n-42)=744\)

\(n=50\)

Answer: D (50)

Nice solution. Correct as always. I was actually thinking of having a ballpark and reach the answer from 43 to 50. Add from 43, 44, 45.. to reach 372, need at least 7 (43x7=301) and less than 9 (43x9=387).

Did you use any intermediary formulas for this part? (n+43)*(n-42)=744 n=50

Nice solution. Correct as always. I was actually thinking of having a ballpark and reach the answer from 43 to 50. Add from 43, 44, 45.. to reach 372, need at least 7 (43x7=301) and less than 9 (43x9=387).

Did you use any intermediary formulas for this part? (n+43)*(n-42)=744 n=50

Sum of consecutive integers or sum of terms in any evenly spaced set (AP): \(\frac{first term+last term}{2}*number of terms\)

In original question: First term: -42 Last term: n Number of terms: 42+n+1=n+43

\(\frac{(n-42)}{2}*(n+43)=372\)

\((n-42)*(n+43)=744\)

Bunel: I understand how you are calculating # of terms given the fact that the sum is +ve. What if the sum were -ve? In that case "n" can be smaller or greater than -42. How do we calculate # of terms in that case. Thank you in advance.

I forgot about AP..... Bunnel you are amazing.......... This is how i tried which is ridiculous as it took arnd 4 mins

Step1: Obviously, we need not worry about numbers till 42 as they will negate with the negative numbers when addition is performed: [strike]-42, -41........0........., 41, 42[/strike], 43...n => 43+44+...+n = 372 Step2: the series can be represented by: 43 + 0, 43+1, 43+2.....43+(x-1)# = 372 # --> Series starting with zero Step3: 43x + [(x-1)x]/2 = 372 --> Solve quadratic equation. Step4: x^2 - x + 86x = 372 => x^2 + 85n -744 = 0 => x^2 + 93x -8x - 744 = 0 => (x+93)(x-8)=0 x=8. Step5: But mind you the series ends with x-1. Hence 43 + (8-1) = 50.

Bunuel, is there a quick way to solve the equation:

744 = (n+43)(n-42)

?

I mean the question is very simple. But the equation looks very tedious to me.

Denote n - 42 = x, where x is a positive integer, and of course, n + 43 = x + 85. You have to solve x(x + 85) = 744. Look for the factorization of 744: 744 = 2 * 2 * 2 * 3 * 31 = 8 * 93 (8 * 90 = 720, so you should try factors around 8 and 90)

Therefore, x = 8 and n = 50.
_________________

PhD in Applied Mathematics Love GMAT Quant questions and running.

If the sum of the consecutive integers from –42 to n inclusive is 372, what is the value of n?

A. 47 B. 48 C. 49 D. 50 E. 51

# of terms =\(42+1+n=(n+43)\)

\(Sum=372=(n+43)*\frac{(n-42)}{2}\)

\(744=(n+43)*(n-42)\)

\(n=50\)

OR

42 terms after zero and 42 terms below zero will total 0. So, our new question will be consecutive integers with first term 43 have sum 372, what is the last term:

\(\frac{43+n}{2}*(n-43+1)=372\)

\((n+43)*(n-42)=744\)

\(n=50\)

Answer: D (50)

Can you please explain how is the number of terms 42 + n +1?

Thanks

gmatclubot

Re: Sum of consecutive integers
[#permalink]
22 Jul 2013, 17:38

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