R
range
The difference between the highest number and the lowest, indicating subtraction. For example, in a list of numbers where the highest number is 25 and the lowest is 17, the range is 25 − 17, or 8 numbers.
reasonable
Possible or "could be."
reasoning strategy
Using known facts and logic to find the answer to a problem. Using ten, using doubles, and making ten are reasoning strategies.
rectangle
A two-dimensional (2-D) shape with 4 sides and 4 square corners. The opposite sides are parallel to each other.
related addition fact
(See fact family.)
related subtraction fact
(See fact family.)
repeated addition
Adding a number to itself two or more times. For example,
rhombus
A 2-dimensional (2-D) shape with four straight sides all having the same length.
rounding
Replacing a number with the nearest convenient or friendly number. Numbers are often rounded to the nearest whole number, ten or hundred. (See also friendly numbers.)
row
In an array, the objects lined up horizontally or going across.
rubric
A set of expectations for student work. Teachers use rubrics to evaluate student work. Rubrics also help students to review and evaluate the quality of their own work or the work of others.
rule machine
An activity where a rule is applied to each input number to get an output number. You may need to determine a missing input, output, or the rule itself.
ruler
A tool for measuring length in inches or centimeters.
