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What is 3 times the difference between S_48 and S_24?

Answer and explanation

Correct answer: 2628

Use the governing formula for the sum of the first k natural numbers: S_k = k(k+1)/2. First calculate S_48 = 48 × 49 / 2 = 24 × 49 = 1176. Next calculate S_24 = 24 × 25 / 2 = 12 × 25 = 300. Their difference is S_48 - S_24 = 1176 - 300 = 876. The question asks for three times this difference, so 3 × 876 = 2628. Hence option B is correct. A useful equivalent interpretation is that S_48 - S_24 is the sum of all terms from 25 through 48; this gives the same value. The other choices indicate an error in one partial sum, a failure to multiply by 3, or an arithmetic mistake in the final step.

Related tags

MathematicsSequencesSum Of Natural NumbersPartial SumsSequences And ProgressionsClass 9 McqSum Of First N Natural Numbers

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