## Estimation in Calculations with Decimal Numbers

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1. Jim spent $3.85 on pens, $4.68 on notebooks, $8.15 on paints and $2.06 on folders.

How much money did he spend altogether?

Estimate your answer without using a calculator.

Then, use a calculator to find the exact sum of money that Jim spent.

Pens = $3.85

Notebooks = $4.68

Paints = $8.15

Folders = $2.06

Notebooks = $4.68

Paints = $8.15

Folders = $2.06

**Using estimation**

Total = $3.85 + $4.68 + $8.15 + $2.06

= $4 + $5 + $8 + $2

= 19

So, Jim spent approximately $19 altogether.

Round off each decimal number to the nearest whole number by studying the digit in the tenths place (first place after the decimal point).

**Using calculator**

Total = $3.85 + $4.68 + $8.15 + $2.06

= $18.74

So, Jim spent exactly $18.74 altogether.

When we compare the two answers, we see that $19 (the estimated answer) is close to $18.74 (the exact answer). So, $19 is a reasonable estimate.

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2. A rectangular plot measures 12.23 m by 9.88 m.

Estimate the area of the plot.

Then using a calculator find the exact area.

Explain the two answers.

Length = 12.23 m

Breadth = 9.88 m

Breadth = 9.88 m

**Using estimation**

Area = 12.23 m × 9.88 m

= 12 m × 10 m

= 120 m

^{2}

So, the area of the plot is approximately 120 m

^{2}.

Round off each decimal number to the nearest whole number by studying the digit in the tenths place (first place after the decimal point).

**Using calculator**

Area = 12.23 m × 9.88 m

= 120.8324 m

^{2}

So, the area of the plot is exactly 120.8324 m

^{2}.

When we compare the two answers, we see that 120 m

^{2}(the estimated answer) is close to 120.8324 m^{2}(the exact answer). So, 120 m^{2}is a reasonable estimate.