Find the missing number: $45 + \square \times 3 = 81$.

Explanation

1. 01

   The multiplication $\square \times 3$ is added to 45, so $\square \times 3 = 81 - 45 = 36$.

2. 02

   Hence $\square = 36 \div 3 = 12$.

3. 03

   Check: $45 + 12 \times 3 = 45 + 36 = 81$.