Multiple choice

A person pays Rs. 975 by monthly instalment, each less than the former by Rs. 5. The first instalment is of Rs. 100. The time by which the entire amount will be paid is

  1. 10 months

  2. 15 months

  3. 14 months

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Monthly installments form an AP: 100, 95, 90, ..., with sum 975. Let n be number of months. S_n = n/2[2a + (n-1)d] = n/2[200 + (n-1)(-5)] = n/2[205 - 5n] = 975. Solving: n(205 - 5n) = 1950, which gives n = 15.

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