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Sum of first 50 odd numbers: 1+3+....+97+99=100x25=2500
(bunch the numbers together, 99+1=100, 97+3=100 and there are 25 such pairs)

Sum of first 50 even numbers: 2+4+...+98+100=102x25=2550
(bunch the numbers together, 100+2=102, 98+4=102 and there are 25 such pairs)

so 2550-2500=50
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I dealt with d first 10 integers (even and odd)
Even
2+4+6+8+10=30
Odd
1+3+5+7+9=25

Even-odd ; 30-25=5

We have 10 more integers in 9 places to go and their differences would always be 5

5 in 10 places makes 50

Hope this helps

Posted from my mobile device
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Sum of first 'n' positive even integers = \(n(n+1)\)
Sum of first 'n' positive odd integers = \(n^2\)

Given:
\(n=50\)
\(X = n(n+1) \)
\(Y = n^2\)

\(X-Y\) = \(n^2 + n - n^2\) = \(n\) = 50 (C)
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If X is the sum of first 50 positive even integers and Y is the sum of first 50 positive odd integers, what is the value of x - y ?

A. 0
B. 25
C. 50
D. 75
E. 100

Sum of terms in a AP =\(N/2\) *(First + Last term)
Sum of first 50 positive even integers = \(50/2\)(2 + 100) = 50 * 51 = X
Sum of first 50 positive odd integers = \(50/2\)(1 + 99) = 50 * 50 = Y
Difference(X -Y) = 50
C
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What is the formula for the quantity of even and odd integer pairs? Thank you.
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nishtil
If x is the sum of the first 50 positive even integers and y is the sum of the first 50 positive odd integers, what is the value of x – y ?

A. 0
B. 25
C. 50
D. 75
E. 100

PS00605

Sum of numbers formula: n/2 (a1 + aN)
Sum of first 50 even numbers: 50/2 (2+100)
Sum of first 50 odd numbers: 50/2 (1+99)
Therefore :
50/2(102-100)
50/2 x 2
50
Hence C
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­Long way and a short way- both quite enjoyable:

­
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JeffTargetTestPrep

nishtil
If X is the sum of first 50 positive even integers and Y is the sum of first 50 positive odd integers, what is the value of x-y?

A. 0
B. 25
C. 50
D. 75
E. 100





 
The sum of the first 50 positive even integers is:

sum = average x quantity

sum = (100 + 2)/2 x 50 = 51 x 50

The sum of the first 50 positive odd integers is:

sum = (99 + 1)/2 x 50 = 50 x 50

Thus, x - y is 51 x 50 - 50 x 50 = 50(51 - 50) = 50.

Alternate solution:

The first 50 positive even integers are: 2, 4, 6, 8, …, 98, 100.

The first 50 positive odd integers are: 1, 3, 5, 7, …, 97, 99.

We see that each even integer is 1 more than its odd counterpart (2 is 1 more than 1, 4 is 1 more than 3, etc). Since there are 50 numbers in each set, the sum of the even integers will be 50 x 1 = 50 more than the sum of the odd integers.

Answer: C
­I understand the Highest +Lowest/ 2 formula (100+2/2=51) to get the average for first 50 positive even/odd integers.

However, how do you definitively *know* that the highest aka 50th number is 100 for the first 50 positive even integers? That is, without listing out all of the first 50 positive integers.... it was easy to plug +2 for the lowest possible even integer but I did not know definitively that the 50th would be 100. I was thinking maybe 98/102 etc without listing all of them out.­ How do you know what to plug in for 50th even/ 50th odd integer­? JeffTargetTestPrep­ Bunuel­
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