Arithmetic Progression Sum

CAT 2025 Slot 3 · Quantitative Ability · Easy · Algebra

In an arithmetic progression, if the sum of fourth, seventh and tenth terms is 99, and the sum of the first fourteen terms is 497, then the sum of first five terms is

Answer

65

Explanation

Let first term be aa and common difference be dd. T4+T7+T10=(a+3d)+(a+6d)+(a+9d)=3a+18d=99  ⟹  a+6d=33T_4 + T_7 + T_{10} = (a+3d) + (a+6d) + (a+9d) = 3a + 18d = 99 \implies a + 6d = 33. S14=142(2a+13d)=7(2a+13d)=497  ⟹  2a+13d=71S_{14} = \frac{14}{2}(2a + 13d) = 7(2a + 13d) = 497 \implies 2a + 13d = 71. Solving these equations: From a=33−6da = 33 - 6d, substitute into second: 2(33−6d)+13d=71  ⟹  66+d=71  ⟹  d=52(33 - 6d) + 13d = 71 \implies 66 + d = 71 \implies d = 5. Then a=33−30=3a = 33 - 30 = 3. Sum of first 5 terms S5=3+8+13+18+23=65S_5 = 3 + 8 + 13 + 18 + 23 = 65.

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