An arithmetic progression has 10 terms. The sum of the last five terms is 85. If the fourth term is 9, then find the sum of all 10 terms.
If the sum of the last 5 terms is 85, and there are 10 terms, then the eighth one is 17 (take 85/5).
And if the fourth term is 9, then the common difference is 2 (pretty simple math).
To find the sum, we just find the median and multiply it by the number of terms.
The median is 12, multiplied by 10, gets us 120.
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