To find out how much Mrs. Perez should write the check for, you need to 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
Now, subtract the cash payment from the total cost:
\[ \text{Balance} = \text{Total cost} - \text{Cash payment} \] \[ \text{Balance} = 986.23 - 497.80 = 488.43 \]
Therefore, Mrs. Perez should write the check for $488.43.