change prefix value "-" to "*"

Hello,

I have Opencart v. 1.4.9.1. and I didnt find file with source:

if ($option_value_query->row['prefix'] == '+') {
$option_price = $option_price + $option_value_query->row['price'];
} elseif ($option_value_query->row['prefix'] == '-') {
$option_price = $option_price - $option_value_query->row['price'];
}

I want to change prefix "-" to "*"
eg:
default product price: 1.00 €
option value 1.5 , prefix=* ----> total price of product: 1.0 * 1.5 = 1.5

where i find this FILE into i want to change this source ?

Thank

Milan