Find (1) If \( 31 \times 25=775 \), then \( 0.31 \times 2.5=\ldots \ldots \) (2) Fifth of \( 15=\ldots \ldots \ldots . \).

To solve the first part, we can use the relationship between whole numbers and decimals. Since \( 0.31 \) is \( 31 \div 100 \) and \( 2.5 \) is \( 25 \div 10 \), we can rewrite the multiplication as: \[ 0.31 \times 2.5 = \left( \frac{31}{100} \right) \times \left( \frac{25}{10} \right) = \frac{31 \times 25}{1000} = \frac{775}{1000} = 0.775. \] For the second part, calculating a fifth of \( 15 \) involves dividing \( 15 \) by \( 5 \): \[ \frac{15}{5} = 3. \] So the answers are: (1) \( 0.31 \times 2.5 = 0.775 \) (2) Fifth of \( 15 = 3 \).