>This is a sample problem out of a math workbook. I get that "estimating" essentially means performing calculations on rounded values, but they are mixing the places they round to in the example below. Why isn't it either 1 x 4 or 0.8 x 4.2? Not quite getting how they are doing that. I thought with rounding, you had to know the place you wanted to round to in order to have the product be valid.
>
>

>Estimate Decimal Products
>-------------------------
>
>Estimate 4.2 x 0.843
>
>  0.843 -------->   0.8
>x   4.2 -------->  x  4
>                   ====
>              about 3.2
>
>Both factors are rounded
>down. The actual product
>is greater than 3.2.
>

I think the principal here is just to round individual numbers to a value that allows you to calculate the result 'in your head'. So the chosen level of rounding simply depends on your ability to perform mental arithmetic....