Final answer:
To rewrite the subtraction expression 3 - (-20) as an addition expression, we convert it to 3 + 20 by changing the subtraction to addition and the sign of the second number.
Step-by-step explanation:
To rewrite the subtraction expression 3 - (-20) using addition without changing the digits, we can use the rule that subtracting a number is the same as adding its opposite. This means we change the subtraction sign to an addition sign and also the sign of the second number. In this case, we take the negative sign off of the -20, making it positive.
Therefore, the expression 3 - (-20) rewritten using addition is 3 + 20.