Q. 194.4( 255 Votes )

200 logs are stac

Answer :

As, the rows are going up, the no of logs are decreasing, 
20, 19, 18, ...,
it's an AP
Suppose 200 logs are arranged in 'n' rows, then
We have;
First term, a = 20,
Common difference, d = - 1 and
Sum of n terms, Sn = No of logs = 200


We know;



200


400= (40 - N +1)

400 = 41N - N2

N2 -41N +400 =0

(N -16)(N –25)


Thus, n = 16 and n = 25


If number of rows is 25 then;


a25 = 20 + 24 x (- 1)


= 20 – 24 = - 4


Since; negative value for number of logs is not possible hence; number of rows = 16


a16 = 20 + 15 x (- 1)


= 20 – 15 = 5


Thus, number of rows = 16 and number of logs in top rows = 5

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