Explanation:
The error in the calculation is that the expression "-30 + (-30)" was simplified as "-60" instead of "-60 + 15".
The correct computation for the expression "-30 + (-30) + 15" is as follows:
-30 + (-30) + 15
= -60 + 15
= -45
Therefore, there is no error in the given calculation. The result -45 is correct.