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You are watching: The number of seats in the first 12 rows

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Re: The variety of seats in the first row of an auditorium is 18<#permalink>
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17 Sep 2017, 21:40
S=n/22a+(n-1)d is the formula for ap as the question is in AP i.e. constantly increasing through 2 eg 18,20,22 and so on Now, a = 1st term which is known=18 d = distinction =21 n =27 so putting a,d,n in the first equation we have the right to know S which is the amount of the development.2 variety of seats in the last row is 70. n=(final term of the series -initial term of the series)/d -1 so n can be calculated and when we understand n , S have the right to be found out . for this reason answer is D
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The auditorium seats are in AP. So sum of full seats shall be Sn=n/2 2a+(n-1) d , wbelow a=18 , d=2(1) says, n=27, so we have the right to uncover the value of Sn. SUFFICIENT(2) claims, l = 70, 70 (tn) deserve to be also written as tn=a+(n-1)d . SUFFICIENTBoth are individually enough. Hence answer D._________________


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Hi All, Certain DS inquiries are really simply "logic" inquiries - interpretation that if you understand the logic connected, then you have the right to correctly answer the question without doing a lot (if any) math. Here, we"re told that the 1st row in an auditorium has 18 seats and each row after has actually 2 more seats than the row that instantly precedes it. Hence.... second row = 20 seats third row = 22 seats fourth row = 24 seats Etc. So, if you understand the row number, then you"ll know the number of seats AND if you recognize the variety of seats, you"ll know the row number. We"re asked for the total variety of rows in the auditorium. 1) The variety of rows of seats in the auditorium is 27. With this Fact, we have the right to absolutely calculate the complete variety of seats (we"d simply need to count them all up). Thanktotally, we don"t actually have to do that math to know that this is sufficient indevelopment to answer the offered question. Fact 1 is SUFFICIENT. (2) The variety of seats in the last row is 70. With this Fact, we could figure out the variety of seats in each of the coming before rows (68, 66, 64, etc.), so we can figure out the complete variety of seats. Just as in Fact 1, we don"t actually need to carry out that math though. Fact 2 is SUFFICIENT. Final Answer: