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Front End Estimation of Difference
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When estimating the difference of two numbers, one
way to estimate the difference is  called
"front-end'
estimation.

In front end estimation of difference,  you subtract
only the first digit of the numbers. The other digits
are turn to zeros.
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> Front End Estimation of Difference < you are here
Example word problem:
There were 435 people signed up to vote in the
election. If 123 people
did note vote, about how
many people voted?
Subtract 123 from 435 using Front End Estimation.
The actual difference is
435 - 123 = 312
About 300 people voted.