Step 1: Conversion from decimal to binary.
To convert 125 to binary, we divide the number by 2 and record the remainders:
\[
125 \div 2 = 62 \quad \text{remainder} \, 1
\]
\[
62 \div 2 = 31 \quad \text{remainder} \, 0
\]
\[
31 \div 2 = 15 \quad \text{remainder} \, 1
\]
\[
15 \div 2 = 7 \quad \text{remainder} \, 1
\]
\[
7 \div 2 = 3 \quad \text{remainder} \, 1
\]
\[
3 \div 2 = 1 \quad \text{remainder} \, 1
\]
\[
1 \div 2 = 0 \quad \text{remainder} \, 1
\]
Reading the remainders from bottom to top, we get \( 1111101_2 \).
Step 2: Conclusion.
Therefore, the binary representation of 125 is 1111101.