An arithmetic sequence has first term 5 and common difference 3. What is the sum of the first 10 terms?

The $10$th term is $a_{10}=5+3(9)=32$. The sum of an arithmetic series is the number of terms times the average of the first and last terms: $S_{10}=10\cdot\frac{5+32}{2}=10\cdot18.5=185$, choice C. Listing the terms $5,8,11,14,17,20,23,26,29,32$ and adding them also gives $185$. Choice D ($320$) multiplies the number of terms by the last term. Choice A ($95$) uses $a_{10}=14$, which is incorrect. Choice B ($140$) comes from averaging with a wrong last term. The sum must be more than $10\cdot5=50$ and less than $10\cdot32=320$.