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# For all even integers n, h(n) is defined to be the sum of the even

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For all even integers n, h(n) is defined to be the sum of the even [#permalink]

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31 Aug 2016, 00:56
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For all even integers n, h(n) is defined to be the sum of the even integers between 2 and n, inclusive. What is the value of h(18)/h(10) ?

(A) 1.8
(B) 3
(C) 6
(D) 18
(E) 60
[Reveal] Spoiler: OA

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Last edited by Bunuel on 31 Aug 2016, 01:18, edited 1 time in total.
RENAMED THE TOPIC.

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Re: What is the value of h(18)/h(10) ? [#permalink]

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31 Aug 2016, 01:38
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sum of n terms can be represented by the formula = $$\frac{n*(n+1)}{2}$$

h(10) :- 2+4+6+8+10 = 2(1+2+3+4+5) = 2 * $$\frac{5*6}{2}$$ = 30

h(18) :- 2+4+6+8+10+12+14+16+18 = 2(1+2+3+4+5+6+7+8+9) = $$\frac{2 * 9 *10}{2}$$ = 90

$$\frac{h(18)}{h(10)}$$ = $$\frac{90}{30}$$ =3

correct answer -$$B$$

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Re: For all even integers n, h(n) is defined to be the sum of the even [#permalink]

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31 Aug 2016, 04:13
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mystiquethinker wrote:
For all even integers n, h(n) is defined to be the sum of the even integers between 2 and n, inclusive. What is the value of h(18)/h(10) ?

(A) 1.8
(B) 3
(C) 6
(D) 18
(E) 60

CONCEPT: When terms are in Arithmetic Progression (A.P.) i.e. terms are equally spaced then

Mean = Median =(First+Last)/2

and Sum = Mean*Number of terms

h(18) = [(2+18)/2]*9 = 90

h(10) = (2+10)/2]*5 = 30

h(18)/h(10) = (90) / (30) = 3
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Re: For all even integers n, h(n) is defined to be the sum of the even [#permalink]

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31 Aug 2016, 06:48
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mystiquethinker wrote:
For all even integers n, h(n) is defined to be the sum of the even integers between 2 and n, inclusive. What is the value of h(18)/h(10) ?

(A) 1.8
(B) 3
(C) 6
(D) 18
(E) 60

We can also apply a nice rule that says (a + b)/c = a/c + b/c AND use some estimation, since the answer choices are nicely spread apart.

h(18)/h(10) = (2+4+6+8+10+12+14+16+18)/(2+4+6+8+10)
= (2+4+6+8+10)/(2+4+6+8+10) + (12+14+16+18)/(2+4+6+8+10)
= 1 + (12+14+16+18)/(2+4+6+8+10)
= 1 + some value around 2 [yes, I'm aware that the fraction evaluates to be exactly 2, but we could actually be quite aggressive in our estimation and still reach the correct answer]
≈ 3

[Reveal] Spoiler:
B

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For all even integers n, h(n) is defined to be the sum of the even [#permalink]

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20 Jul 2017, 12:48
Hi,
Could I apply the following?

$$h(18)/h(10)$$ simplified is $$h(9)/h(5)$$, so the even numbers are $$(2+4+6+8)/(2+4)$$ = 3

Thanks

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For all even integers n, h(n) is defined to be the sum of the even [#permalink]

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22 Aug 2017, 08:05
Esguitar wrote:
Hi,
Could I apply the following?

$$h(18)/h(10)$$ simplified is $$h(9)/h(5)$$, so the even numbers are $$(2+4+6+8)/(2+4)$$ = 3

Thanks

h(10) = 2+4+6+8+10 = 30
h(5) = 2+4 =6
so h(10) can't = 2 x h(5), you can't simplify like that.

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Re: For all even integers n, h(n) is defined to be the sum of the even [#permalink]

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22 Aug 2017, 09:46
The number are in AP
we can use n/2(2a + (n-1)d)
we will get sum of n even terms ---( a = 2 , d = 2) =n(n+1)
Now 2 to 18 we have 9 terms
2 to 10 we have 5 terms
9*10 / 5*6
we will get 3

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Re: For all even integers n, h(n) is defined to be the sum of the even [#permalink]

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24 Aug 2017, 15:05
mystiquethinker wrote:
For all even integers n, h(n) is defined to be the sum of the even integers between 2 and n, inclusive. What is the value of h(18)/h(10) ?

(A) 1.8
(B) 3
(C) 6
(D) 18
(E) 60

First, notice that h(18) will be the sum of the even integers between 2 and 18 inclusive. We can thus calculate h(18) using the formula sum = avg x quantity.

First, find the average of this evenly spaced set of numbers:

avg = (18 + 2)/2 = 10

Now calculate how many numbers are in this set:

quantity = (18 - 2)/2 + 1 = 9

Now we use the formula sum = avg x quantity:

sum = (10)(9)

sum = 90 = h(18)

Next we can determine h(10) using the same procedure we used to calculate h(18):

avg = (10 + 2)/2 = 6

quantity = (10 - 2)/2 + 1 = 5

sum = 30

Thus, h(18)/h(10) = 90/30 = 3

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Re: For all even integers n, h(n) is defined to be the sum of the even   [#permalink] 24 Aug 2017, 15:05
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