Logs are stacked with 20 in the bottom row, 19 in the row above it, 18 in the next, and so on. If 200 logs are stacked in this way, the number of rows is:

Here $a=20$ and $d=-1$, so Sₙ = $n/2[40-(n-1)]=n(41-n)/2=200$ gives $n^2-41n+400=0$. Since √$(1681-1600)=9,n$ = (41 ± 9)/2 = 25 or 16. A stack of 25 rows is impossible (the count would fall to zero and below), so $n=16$, and the top row holds 20 − 15 = 5 logs. Check: S₁₆ = $8(40-15)=200$.