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Question

A shopkeeper buys a number of books for Rs.80. If he had bought 4 more for the same amount each book would have cost Rs.1 less. How many books did he buy?


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Solution

Step 1: Finding the cost of each book

Let a shopkeeper buys a 'x' number of books for Rs.80.
Cost of each book =80x
If he bought 4 more books that is x+4 for the same amount
then cost of each book =80x+4

Step 2: Finding the number of books he bought

each book would have cost Rs.1 less, then we get
80x-80x+4=180x+320-80xx(x+4)=1x2+4x-320=0x2+(20-16)x-320=0x2+20x-16x-320=0x(x+20)-16(x+20)=0(x-16)(x+20)=0x=16,-20
Number of books cannot be negative

Therefore number of books shopkeeper buy is 16


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