Another Peek Beyond the Point | IT

Question 4

Can you give a simple rule to divide any number by a number of the form 1 followed by zeroes — 10, 100, 1000, etc.? For example, $\dfrac{123}{10}$ , $\dfrac{24}{100}$ or $\dfrac{678}{1000}$ ? Look for a pattern in the previous problems.

Solution

Understand the Question

When dividing a whole number or decimal by numbers like $10$ , $100$ , or $1000$ , the digits remain the same while their place values decrease.
Each zero in the divisor represents a division by $10$ , which shifts the decimal point one place to the left.
Counting the number of zeroes in the divisor gives the exact number of places the decimal point must move to the left.

Step 1 · Divide by 10

For $\dfrac{123}{10}$ , write $123$ as $123.0$ . Diagram 1

Since $10$ has $1$ zero, move the decimal point $1$ place to the left:

\begin{aligned} 123 \div 10 &= 123.0 \div 10 \\ &= 12.3 \end{aligned}

Step 2 · Divide by 100

For $\dfrac{24}{100}$ , write $24$ as $24.0$ . Diagram 2

Since $100$ has $2$ zeroes, move the decimal point $2$ places to the left:

\begin{aligned} 24 \div 100 &= 24.0 \div 100 \\ &= 0.24 \end{aligned}

Step 3 · Divide by 1000

For $\dfrac{678}{1000}$ , write $678$ as $678.0$ . Diagram 3

Since $1000$ has $3$ zeroes, move the decimal point $3$ places to the left:

\begin{aligned} 678 \div 1000 &= 678.0 \div 1000 \\ &= 0.678 \end{aligned}

Step 4 · State the General Rule

Observing the pattern:

Dividing by $10$ ( $1$ zero) $\rightarrow$ Shift decimal point $1$ place left.
Dividing by $100$ ( $2$ zeroes) $\rightarrow$ Shift decimal point $2$ places left.
Dividing by $1000$ ( $3$ zeroes) $\rightarrow$ Shift decimal point $3$ places left.

Rule: To divide any number by $10, 100, 1000, \dots$ , shift the decimal point to the left by as many places as there are zeroes in the divisor.

Answer

To divide a number by $10$ , $100$ , $1000$ , etc., move the decimal point to the left by as many places as the number of zeroes in the divisor.

Common Mistakes

Direction Confusion: Moving the decimal point to the right instead of the left. Shifting right multiplies the number, whereas division always makes the number smaller.
Missing Placeholders: Forgetting to add leading zeroes when shifting past the available digits (e.g., $5 \div 100 = 0.05$ , not $0.5$ ).

More questions in IT

Jonali and Pallabi play a game. Jonali says a fraction and Pallabi gives the equivalent decimal. Write Pallabi's answer in the blank spaces.

Jonali goes to the market to buy spices. She purchases $50\text{ g}$ of Cinnamon, $100\text{ g}$ of Cumin seeds, $25\text{ g}$ of Cardamom and $250\text{ g}$ of Pepper. Express each of the quantities in kilograms by writing them in terms of fractions as well as decimals.

Write the following fractions as a sum of fractions and also as decimals:

Can the product of two decimals be a natural number?

Can the product of a decimal and a natural number be a natural number?

Observe the number of digits after the decimal point in the multiplier, the multiplicand and the product. Also note the number of zeroes in the denominator.

Suppose we know that $596 \times 248 = 147808$ , can you immediately write down the product of $5.96 \times 24.8$ ?

Context: Observe the number of digits after the decimal point in the multiplier, the multiplicand and the product. Also note the number of zeroes in the denominator.

Q. By looking at the above examples, can you frame a rule to multiply two decimals?

Q10

When is the product of two decimals greater than both the numbers? When is it less than both the numbers?

Q11

What is $0.039 \text{ m}$ in centimetres and millimetres?

Q12

Complete the table by dividing the decimals:

Q13

Can you find the quotients of $10 \div 9$ , and $100 \div 11$ ?

Q14

Context: Let us consider the number $142857$ that arose when dividing $1$ by $7$ . Multiply $142857$ by numbers from $1$ to $6$ .

Q. What are the products? What do you notice?

Multiply $142857$ by $7$ . What do you observe?

Q15

To find one such number, you can find $1 \div 17$ in decimal, and use the repeating block of digits.

Q16

Will the quotient be always greater than the dividend when the divisor is a decimal? Try it out with different values of the divisor.

Describe the relationship between the dividend, divisor, and the quotient. Create a table for capturing this relationship in different situations, like we did for multiplication.

Q17

What pattern do you observe? Why are 2 and 5 related in this way?

Q18

Do you know which month has this extra day?

Q19

With this new scheme of adding one extra day every 4th year, what is the number of days in 100 calendar years? Can you write an expression to calculate that number?

Q20

How many years are divisible by 4 in 100 years?

Q21

Can you form different expressions for the same question?

Q22

Context: The calendar designers decided that they will not add 1 extra day in every hundredth year.

Q. Can you write an expression for the number of days in 100 calendar years with this new adjustment?

Q23

With this final scheme of leap years can you calculate the number of calendar days in 10,000 years and the number of actual days the Earth will take to make 10,000 revolutions around the Sun? What is the difference? If there is a big difference, can you suggest a way to fix this problem?

Q24

Do you wonder how people figured out that the Earth completes one revolution around the Sun in exactly 364.2422 days?