Final answer:
The annual interest rate for Mateo's savings account in the given expression is represented by the value 0.02, which indicates a 2% annual interest rate.
Step-by-step explanation:
When determining the balance of a savings account with compound interest, the expression used typically takes the form of the principal amount multiplied by (1 + annual interest rate)^t, where t represents the number of years the money is invested. In the expression Mateo uses, the value which represents the annual interest rate for his savings account is mentioned as 0.02. This signifies a 2% annual interest rate. Without the full correct expression provided by the student, we are inferring this from the context given, where the interest rate is expressed in decimal form and is typically a number less than 1 (for rates below 100%). It should also be inserted into the exponential formula as part of the base that is raised to the power of t, which stands for the passage of time in years.