In an arithmetic progression the first term is 5 and the common difference is 7. How many terms add up to 920?
Answer and explanation
Correct answer: B. 16
- Step 1: Use S = n/2 × [2a + (n − 1)d], which gives 7 n² + 3 n − 1840 = 0. Step 2. The positive root is n = 16; check: 16/2 × [10 + 15 × 7] = 920. Answer: 16. Why not the others: 17 is what you get if you count one term too many. 15 is what you get if you count one term too few. 18 is what you get if you divide the sum by the last term.
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