Estimating differences

In this lesson, we will learn:

• How to estimate the answer to subtraction statements
• The two methods for estimating differences: front-end estimation and estimation by rounding
• What to do if you get an estimated difference of zeros
• How to check and compare your estimated differences with the exact answer

• An estimation is a rough calculation of what the exact answer could be around. It is less exact but easier (faster) to calculate!


• When estimating, it is helpful to remember the rules on how to round numbers. If the number to the right (of what you are rounding to) is:
• Greater than 5 ($\geq$ 5), round UP
• Less than 5 (< 5), round DOWN
• For mixed fractions, look at the fraction portion to round to the nearest whole number. If the fraction is $\geq$ $\frac{1}{2}$, round UP. If the fraction is < $\frac{1}{2}$ , round DOWN.


• Two methods to estimate differences: front-end estimation and estimation by rounding
• Front-End Estimation:
1. Subtract the front digits
• The front digit is the greatest place value out of all your addends
• Subtracting mixed fractions: subtract the whole number parts only
2. Write zeroes
• All the other digits of the answer become zero; skip this step for mixed fractions
• *Note: you do NOT need to adjust the back digits for estimating differences; you only need to adjust when estimating sums


• Estimation by Rounding:
1. Round 
• Round to the greatest place value of the smallest number out of all your addends
• If you are subtracting mixed fractions, round to the nearest whole number
2. Subtract the rounded numbers
• If you get an estimated difference of zero, you must start over by rounding to the next place value smaller (to the right)