A man arranges to pay off a debt of Rs3600 by 40 annual installment which form an arithmetic series. When 30 of the installments are paid he dies leaving one-third of the debt unpaid. Find the value of the first installment.
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Solution
S40=3600,S30=3600−13[3600]=2400,
because after paying 30 instalments 13rd of debt is still left. The given two equations give 2a+39d=180 and 2a+29d=160. Solving them, we get d=2 and a=51, i.e., the value of first instalment is Rs. 51.