Definition
Missing addend
A missing addend is the unknown number that must be added to make the stated sum.
What it means
In an equation such as 8 + □ = 15, the box is one of the parts being added. Find it by asking what joins 8 to make 15. You can move forward from 8, complete a number bond, or undo the addition with 15 − 8.
Remember it
Known part + missing part = whole.
Example
In 13 + □ = 21, subtract 21 − 13 = 8. The missing addend is 8 because 13 + 8 = 21.
Watch for
The missing addend is not the sum: in 13 + □ = 21, the sum is 21.
Common mix-up
In □ + 8 = 15, the blank is still an addend even though it comes first.
Exact meaning
A missing addend is an unknown number in an addition equation that, when added to a known addend, gives the stated sum.