Rounding numbers allows one to estimate calculations
mentally. One method is to round both numbers to the
same place value, then complete the calculation using the
rounded numbers. When you are learning estimation
skills, it is great to check your estimation on a calculator:
The next example explains the method of round
numbers to one non-zero digit.
Estimating the Quotient of Decimals
THE QUICKEST WAY to estimate the quotient
of two decimals is to round both the divisor and dividend
to one non-zero digit, and then divide the rounded decimals.
EXAMPLE: Estimate the quotient of 5.239 divided by 0.51
The first thing to do is round the two decimals so that
each has one significant digit (non-zero digit). Rounding
the divisor to one non-zero digit leaves 0.5 , and rounding
the dividend to one non-zero digit leaves 5.
Now the division problem looks like this:
5 ¸ 0.5
The decimal point in the divisor needs to
be moved one place to the right, so that it is all the way
to the right. Similarly the decimal point in the dividend
is moved one place to the right; we need to add a 0 to the
right of the 5 so this can be done. We can also get rid
of the extraneous 0 to the left of the 5 in the divisor.
The estimate concludes with:
50 ¸ 5 = 10
modified source: BMCC
Math Tutorials -- Decimals -- Division of Decimals