Question 1132502
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The top ("first") layer contains 20 logs.<br>
Each layer farther down contains 1 more log than the above layer; that means the 25th (last) layer contains 20+24 = 44 logs.<br>
The total number of logs is then<br>
20+21+22+...+43+44<br>
Since this is an arithmetic sequence, the average number of logs in each layer is the average of the numbers in the first and last layers: (20+44)/2 = 32.<br>
Then the total number of logs in the pile is the number of layers, times the average number of logs in each layer: 25*32 = 800.