A Number Trick!

Resource Info

Basic Information

Here is an interesting way one can multiply small and big numbers. Also known as Russian multiplication, the peasants are said to have devised this method.

Duration: 
02 hours 00 mins
Introduction: 

Multiplication and division can sometimes turn out to be tedious, and when children are working on it, the more so. Here are some easy ways one can multiply small and big numbers. This could be a fun activity and when played as a game, the interest is maintained.

Objective: 
  • Children can learn while playing a game
  • Group skills are reinforced
Activity Steps: 

A number trick

Here is an interesting way of multiplying numbers small and large knowing only multiplication and division by two. Let’s say we want to multiply 75 by 38. Make two columns on your page.

Take the first (large) numbers 75 on the left column, and the second (smaller) number 38 on the right.

Divide 75 by two: you get 37.5. Ignoring the fractional part, write down the whole number just below 75, and continue to halve the dividend by two until you get to the lowest whole number, i.e. 1.

Divide the number by 2

Multiply the number by 2

75

38

37

76

18

152

9

304

4

608

2

1216

1

2432

 

 

 

 

 

 

 

 

 

 

 

 

Multiply 38 by 2, writing down the product under 38. Continue to multiply the product by two until you have as many rows on the column as you do on the other.

Divide the number by 2

Multiply the number by 2

75

38

37

76

18

152

9

304

4

608

2

1216

1

2432

 

 

 

 

 

Very Important: On 4th row we have a pair of even numbers (18 & 152). Strike out all such even number pairs in any rows.

 

 

 

 

 

 

Now add only the numbers that remain on the right column (38 + 76 + 304 + 2432) to get the answer  2850!

Let’s take another example: 84 x 28

Divide the number by 2

Multiply the number by 2

84

28

42

56

21

112

10

224

5

448

2

896

1

1792

 

 

 

 

 

 

Very Important: On the 1st row we have a pair of even numbers (84 & 28). Strike out all such even number pairs in any rows.

 

 

 

 

 

Now add 112 + 448 + 1792 to get the answer: 2352!

This activity can even be played as a game in the classroom. The children not only enjoy the game but multiplication and division become interesting to them.

This article first appeared in Teacher Plus Vol.II, Issue No2, March-April 2004 and has been adapted here with changes.

Comments

sandan's picture

the activity on the whole is good , it really shows a new of approaching a math topic. but this activity involves one or two competencies in which the pupil must ve a predefined knowledge on that, before approaching this method, because i m working in a rural school where basic skills lack in...... on considering all those , its a nice activity to indulge in................

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