- A21978
- B22011
- C20988
- D21500
Step-by-Step Solution
First multiple of 9 above 300: 306. Last below 700: 693. .
Answer: A) 21978
Find the sum of all multiples of 9 lying between 300 and 700.
Class 10Arithmetic ProgressionsAlgebra — Sequences
Key idea: Common AP word problems
**Salaries**: 'Person earns ₹X in year 1 with annual increase of ₹Y. Find salary in year n / total over n years.' → AP with a = X, d = Y.
**Stadium seating**: 'Front row has 30 seats; each row 4 more. 25 rows. Total seats?' → AP with a = 30, d = 4. S₂₅ = ?
**Loans/installments**: equal monthly payments form an AP.
**Number patterns**: 'Multiples of 5 between 100 and 500: how many? Sum?' → AP with a = 100, d = 5, last = 500.
Always identify a, d, and what's asked (term aₙ or sum Sₙ). Check whether n is the COUNT or the POSITION.
**Stadium seating**: 'Front row has 30 seats; each row 4 more. 25 rows. Total seats?' → AP with a = 30, d = 4. S₂₅ = ?
**Loans/installments**: equal monthly payments form an AP.
**Number patterns**: 'Multiples of 5 between 100 and 500: how many? Sum?' → AP with a = 100, d = 5, last = 500.
Always identify a, d, and what's asked (term aₙ or sum Sₙ). Check whether n is the COUNT or the POSITION.
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