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.