To determine the sign of the product of \(-10.4\) and \(-78.2\), we can analyze the signs of the numbers involved:
- \(-10.4\) is negative.
- \(-78.2\) is also negative.
The product of two negative numbers is positive. Therefore, the product of \(-10.4\) and \(-78.2\) is positive.
So, the sign of the product is \(+\).