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, S_{n} = No of logs = 200

We know;

200

400= (40 - N +1)

400 = 41N - N^{2}

N^{2} -41N +400 =0

(N -16)(N –25)

Thus, n = 16 and n = 25

If number of rows is 25 then;

a_{25} = 20 + 24 x (- 1)

= 20 – 24 = - 4

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

a_{16} = 20 + 15 x (- 1)

= 20 – 15 = 5

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

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