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28.  Shamshad Ali buys a scooter for Rs 22000. He pays Rs 4000 cash and agrees to pay the balance in annual instalment of Rs 1000 plus 10% interest on the unpaid amount. How much will the scooter cost him?

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Given: Shamshad Ali buys a scooter for Rs 22000.

Therefore , unpaid amount = 22000-4000=Rs. 18000

According to the given condition, interest paid annually is

10% of 18000,10% of 17000,10% of 16000,......................10% of 1000.

Thus, total interest to be paid 

                                   =10\%of\, 18000+10\%of\, 17000+.............10\%of\, 1000

                                     =10\%of\, (18000+ 17000+.............+ 1000)

                                    =10\%of\, (1000+ 2000+.............+ 18000)

 

Here, 1000, 2000,.............17000,18000 is a AP with first term =a=1000 and common difference =d = 1000

We know that      a_n=a+(n-1)d

                   \Rightarrow 18000=1000+(n-1)1000

                   \Rightarrow 17000=(n-1)1000

                  \Rightarrow 17=(n-1)

                 \Rightarrow n=17+1=18

Sum of AP:

S_1_8=\frac{18}{2}\left [ 2(1000)+(18-1)1000 \right ]

         =9\left [ 2000+17000 \right ]

         =9\left [ 19000 \right ]

         =171000

Thus, interest to be paid : 

                                       =10\%of\, (1000+ 2000+.............+ 18000)

                                        =10\%of\, ( 171000)

                                        =Rs. 17100

Thus, cost of tractor = Rs. 22000+ Rs. 17100 = Rs. 39100

Posted by

seema garhwal

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