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The sum of the first n terms of an arithmetic sequence is 210. If the first term is 5 and the common difference is 3, find n.
But sum must be integer. Try exact values.
But assume problem expects integer — perhaps typo?
Recheck problem: perhaps sum is 210 is correct, but try n = 10: 185, n = 12: 258 — no match.
But wait — maybe the sequence starts at n=1? Yes.
Alternatively, perhaps arithmetic series with a = 5, d = 3, find n such that sum = 210.
So no integer n. But in math olympiad, problems usually have integer answers.
Wait — maybe the sum is 210 and d = 3, a = 5, solve numerically?
Or perhaps its geometric? No, says arithmetic.
Alternatively, accept the quadratic: