On the number line, −15 is to the ______ of zero.
left
Step 1: A number line increases to the right and decreases to the left.
Step 2: \(0\) is the reference point in the middle.
Step 3: Negative numbers are less than \(0\).
Step 4: \(-15\) is a negative number.
Step 5: All negative numbers are placed to the left of \(0\).
Conclusion: \(-15\) is to the left of \(0\).