Explanation

By observing the pattern:

• 18 → 10 (subtract 8)
• 10 → 6 (subtract 4)
• 6 → 4 (subtract 2)
• 4 → 3 (subtract 1)
• 3 → ?

Following the pattern of halving the subtraction amount, the next step would be subtracting 0.5, giving:
3 - 0.5 = 2.5