#### Answer

variable is d

#### Work Step by Step

A variable is a symbol that represents a number that is unknown.
In the expression 21 + d, d is the only symbol (letter) that represents a number we don't know yet.
Therefore, d is the variable

Published by
Pearson

ISBN 10:
0321955048

ISBN 13:
978-0-32195-504-3

variable is d

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