200 logs are stac

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 - 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