To find out how much Mrs. Perez should write the check for, we need to subtract the amount she paid in cash from the total cost of the refrigerator.
Total cost of the refrigerator: $1368.25
Cash payment: $497.80
Now, subtract the cash payment from the total cost:
$1368.25 - $497.80 = $870.45
Therefore, Mrs. Perez should write the check for $870.45.