What is 2.5 × 0.4?

Multiply as if integers: 25 × 4 = 100. Then count decimal places in the original numbers: 2.5 has 1 decimal place; 0.4 has 1 decimal place. Total decimal places: 2. Place the decimal in the answer: 1.00 or 1.0. So 2.5 × 0.4 = 1.0. Quick check: 2.5 is between 2 and 3; 0.4 is less than half. So the answer should be a bit less than half of 2.5, which is about 1.25 — close to 1.0 ✓. Common decimal multiplication mistakes: misplacing the decimal point, or just guessing. The method 'count total decimal places in inputs, put that many in the answer' always works. Another approach: convert to fractions. 2.5 = 5/2; 0.4 = 2/5; (5/2) × (2/5) = 10/10 = 1. Note: when multiplying by a number less than 1, the result is smaller than the original; when multiplying by a number greater than 1, the result is larger.