Estimation in Calculations with Decimal Numbers

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
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.

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
Using estimation
Area  =  12.23 m  ×  9.88 m
=  12 m  ×  10 m
=  120 m2
So, the area of the plot is approximately 120 m2.
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 m2
So, the area of the plot is exactly 120.8324 m2.
When we compare the two answers, we see that 120 m2 (the estimated answer) is close to 120.8324 m2 (the exact answer). So, 120 m2 is a reasonable estimate.