Mary solved the following problem:

[tex]
\[
\begin{array}{l}
57-6^2+(10+2) \\
57-6^2+12 \\
57-36+12 \\
57-48 \\
9
\end{array}
\]
[/tex]

Which of these statements is true?

A. It is wrong because [tex]6^2[/tex] is 12.
B. It is correct.
C. It is wrong because the subtraction should have been done before the addition.
D. It is wrong because [tex]57-48[/tex] is 11.

We start with the expression:

[tex]$$
57-6^2+(10+2)
$$[/tex]

Step 1. Evaluate the exponent.
Compute [tex]$6^2$[/tex]:

[tex]$$
6^2 = 36.
$$[/tex]

So the expression becomes:

[tex]$$
57 - 36 + (10+2).
$$[/tex]

Step 2. Evaluate the parentheses.
Compute [tex]$(10+2)$[/tex]:

[tex]$$
10+2 = 12.
$$[/tex]

Now the expression is:

[tex]$$
57 - 36 + 12.
$$[/tex]

Step 3. Perform the subtraction and addition from left to right.
The correct order for addition and subtraction is to work from left to right. First, subtract:

[tex]$$
57 - 36 = 21.
$$[/tex]

Then add:

[tex]$$
21 + 12 = 33.
$$[/tex]

Mary's error.
Mary’s work mistakenly grouped the subtraction with the addition differently by calculating:

[tex]$$
57 - (36+12) = 57 - 48 = 9.
$$[/tex]

This is incorrect because subtraction and addition should be performed left to right, not by grouping the [tex]$-36$[/tex] and [tex]$+12$[/tex].

Conclusion.
The mistake occurred because the subtraction should have been done before the addition. The correct explanation is:

[tex]$$\textbf{It is wrong because the subtraction should have been done before the addition.}$$[/tex]

Thus, the correct statement is option 3.