R
reasonable
Possible or "could be." For example, it is reasonable to say a first grader is 100 links tall, but it is not reasonable to say a first-grader is 3 links tall.
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.
rectangular prism
A three-dimensional (3-D) shape whose faces are all rectangles.
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,
repeating pattern
A pattern in which the pattern unit is repeated two or more times without change. (See also pattern unit.)
rhombus
A 2-dimensional (2-D) shape with four straight sides all having the same length.
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.
