Solve 15?15=0: Finding the Correct Operation Sign

To solve this problem, we need to determine which arithmetic operation between two identical numbers, 15, results in zero.

• Step 1: Consider the operation of subtraction. For any number $a$, the subtraction $a - a$ equals zero.
• Step 2: Apply this to the problem. Subtracting 15 from 15 is $15 - 15$, which equals 0. This satisfies the condition given in the problem.
• Step 3: Verify against other operations:
  • Addition: $15 + 15 = 30$, not zero.
  • Multiplication: $15 \times 15 = 225$, not zero.
  • Division: $15 \div 15 = 1$, not zero.

Since subtraction is the only operation resulting in zero, the correct sign is a minus sign.

Therefore, the correct answer is $-$.