To find out how much Mrs. Perez should write the check for, subtract the amount she paid in cash from the total cost of the washing machine.
Total cost of the washing machine: $986.23
Amount paid in cash: $497.80
To find the balance: \[ \text{Balance} = \text{Total cost} - \text{Amount paid in cash} \] \[ \text{Balance} = 986.23 - 497.80 = 488.43 \]
Therefore, Mrs. Perez should write the check for $488.43.