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# If g(x) = 2x^2 + 20, what is g(21)?

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VP
Joined: 23 Feb 2015
Posts: 1258
If g(x) = 2x^2 + 20, what is g(21)?  [#permalink]

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Updated on: 17 Feb 2019, 21:38
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5% (low)

Question Stats:

96% (00:43) correct 4% (00:32) wrong based on 28 sessions

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If $$g(x) = 2x^2 + 20$$, what is g(21)?

(A) 12
(B) 17
(C) 902
(D) 288
(E) 305

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Originally posted by Asad on 17 Feb 2019, 11:07.
Last edited by Bunuel on 17 Feb 2019, 21:38, edited 1 time in total.
Renamed the topic.
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Joined: 15 Sep 2017
Posts: 44
Location: United States (IL)
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Re: If g(x) = 2x^2 + 20, what is g(21)?  [#permalink]

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Updated on: 17 Feb 2019, 16:38
g(x)=2x^2 +20, then

g(21)= 2 * (21^2)+20

= 902

Ans is C
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Tashin

Originally posted by Tashin Azad on 17 Feb 2019, 11:29.
Last edited by Tashin Azad on 17 Feb 2019, 16:38, edited 1 time in total.
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Re: If g(x) = 2x^2 + 20, what is g(21)?  [#permalink]

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17 Feb 2019, 11:44
If $$g(x) = 2x^2 + 20$$, what is g(21)?

(A) 12
(B) 17
(C) 902
(D) 288
(E) 305

g(21) = $$2 * {21}^2 + 20$$
= 2[ 441 +10]
= 2 * 451
= 902

C
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Re: If g(x) = 2x^2 + 20, what is g(21)?  [#permalink]

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17 Feb 2019, 21:11
1
1
$$g(21)=2(21)^2+20$$
There is no need to actually solve this.
$$21^2$$=some number with a units digit of 1. That number is multiplied by 2 so the units digit is 2. Adding 20 to that number doesn't change the units digit. Only two answer choices end in 2 and it clearly isn't A.

(C) 902
VP
Joined: 23 Feb 2015
Posts: 1258
Re: If g(x) = 2x^2 + 20, what is g(21)?  [#permalink]

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18 Feb 2019, 00:40
philipssonicare wrote:
$$g(21)=2(21)^2+20$$
There is no need to actually solve this.
$$21^2$$=some number with a units digit of 1. That number is multiplied by 2 so the units digit is 2. Adding 20 to that number doesn't change the units digit. Only two answer choices end in 2 and it clearly isn't A.

(C) 902

This one is a very perfect way to solve this problem.
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Intern
Joined: 08 Aug 2018
Posts: 2
Location: India
Concentration: Nonprofit, Strategy
WE: Web Development (Computer Software)
Re: If g(x) = 2x^2 + 20, what is g(21)?  [#permalink]

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18 Feb 2019, 02:39
g(21)=2*21*21+20
=2*XX1 +20 (last digit will be 1)
=XX2+20
=XX2

Only answer with last digit 2 is C.
Re: If g(x) = 2x^2 + 20, what is g(21)?   [#permalink] 18 Feb 2019, 02:39
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