4.1 INTRODUCTION

    One day, Siri went to a book shop with her father and bought 3 notebooks and 2 pens. Her father paid `80 for them. Her friend Laxmi liked the notebooks and pens. So she too bought 4 notebooks and 3 pens of the same kind for `110. Now her classmates Rubina liked the pens whereas Joseph liked the notebooks. They asked Siri the cost of the pen and the notebook separately. But, Siri did not know the costs separately. How will they find the costs of these items?

    In this example, the cost of a notebook and a pen are not known. These are unknown quantities. We come across many such situations in our day-to-day life.

THINK - DISCUSS

Two situations are given below:,

(i) The cost of 1kg potatoes and 2kg tomatoes was `30 on a certain day. After two days, the cost of 2kg potatoes and 4kg tomatoes was found to be `66.

(ii) The coach of a cricket team of M.K.Nagar High School buys 3 bats and 6 balls for `3900. Later he buys one more bat and 2 balls for `1300.

Identify the unknowns in each situation. We observe that there are two unknowns in each case.

4.1.1 HOW DO WE FIND UNKNOWN QUANTITIES?

 In the introduction, Siri bought 3 notebooks and 2 pens for `80. How can we find the cost of a notebook or the cost of a pen?

Rubina and Joseph tried to guess. Rubina said that price of each notebook could be `25. Then three notebooks would cost `75 and the two pens would cost `5. In that cases each pen could be for `2.50.

Joseph felt that `2.50 for one pen was too less. In his opinion, it should be at least `16. Then the price of each notebook would also be `16.



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