Estimation : Estimation is another word for finding a close but not necessarily exact solution to an arithmetic problem.

Hundredths digit : The hundredths digit digit is the second digit after the decimal point. It counts multiples of $\frac{1}{100}.$

Number : A number $x$ is a mathematical symbol representing a quantity.

Rounding : Rounding is the operation of changing a number to a number that is close to the original number. Rounded numbers are often used to do quick rough calculations for estimation purposes. For example rounding $4549$ to the nearest thousand results in 5000, rounding the same number to the nearest hundred results in 4500. In general one rounds up if the digit after the digit we try to round to is 5 or more, otherwise one rounds down.

Significant figure : Rounding a number to k significant figures means that one leaves the first $k-1$ digits unchanged and rounds the k-th digit up if the (k+1)-th digit is at least 5 and rounds it down otherwise. For example rounding the number 1234 to 2 significant digits results in 1200.

Thousandths digit : The thousandths digit is the third digit after the decimal point. It counts multiples of $\frac{1}{1,000}.$

Round up the first number if the hundredths digit is at least 5 otherwise round down. Round up the second number if the thousandths digit is at least 5, otherwise round down.