NCERT Book Class 7 Maths Chapter 4 Simple Equations

SIMPLE EQUATIONS 59

Chapter 4
Simple
Equations

4.1 A MIND- READING GAME !
The teacher has said that she would be starting a new chapter in
mathematics and it is going to be simple equations. Appu, Sarita
and Ameena have revised what they learnt in algebra chapter in
Class VI. Have you? Appu, Sarita and Ameena are excited because
they have constructed a game which they call mind reader and they
want to present it to the whole class.
The teacher appreciates their enthusiasm and invites them to present their game. Ameena
begins; she asks Sara to think of a number, multiply it by 4 and add 5 to the product. Then,
she asks Sara to tell the result. She says it is 65. Ameena instantly declares that the number
Sara had thought of is 15. Sara nods. The whole class including Sara is surprised.
It is Appu’s turn now. He asks Balu to think of a number, multiply it by 10 and subtract
20 from the product. He then asks Balu what his result is? Balu says it is 50. Appu
immediately tells the number thought by Balu. It is 7, Balu confirms it.
Everybody wants to know how the ‘mind reader’ presented by Appu, Sarita and
Ameena works. Can you see how it works? After studying this chapter and chapter 12,
you will very well know how the game works.

4.2 SETTING UP OF AN EQUATION
Let us take Ameena’s example. Ameena asks Sara to think of a number. Ameena does not
know the number. For her, it could be anything 1, 2, 3, . . ., 11, . . . , 100, . . . . Let us
denote this unknown number by a letter, say x. You may use y or t or some other letter in
place of x. It does not matter which letter we use to denote the unknown number Sara has
thought of. When Sara multiplies the number by 4, she gets 4x. She then adds 5 to the
product, which gives 4x + 5. The value of (4x + 5) depends on the value of x. Thus
if x = 1, 4x + 5 = 4 ×1 + 5 = 9. This means that if Sara had 1 in her mind, her result would
have been 9. Similarly, if she thought of 5, then for x = 5, 4x + 5 = 4 × 5 + 5 = 25; Thus
if Sara had chosen 5, the result would have been 25.