125 + (68+17) = (125 + ____) + 17
68
Idea: We will use the associative property of addition.
The rule says:
\[ a + (b + c) = (a + b) + c \]
Match the numbers:
\[ a = 125,\quad b = 68,\quad c = 17 \]
Start with the left side:
\[ 125 + (68 + 17) \]
Apply the rule:
\[ 125 + (68 + 17) = (125 + 68) + 17 \]
Now compare with the given form:
\[ (125 + \_\_\_) + 17 \]
The blank must be:
\[ \boxed{68} \]
Quick check:
Left side: \[68 + 17 = 85,\quad 125 + 85 = 210\]
Right side: \[125 + 68 = 193,\quad 193 + 17 = 210\]
Both sides are \(210\). So the answer is correct.