How to multiply a three digit number by a one digit number
(e.g. 312 * 3).

• Place one number above the other so that the ones' place digits are lined up. Draw a line under the bottom number.

312
  3

• Multiply the two ones' place digits. (2 * 3 = 6). Place the six below the line in the ones' place column.

  
  312
    3
    6
  

• Multiply the digit in the tens' place column (1) by the digit in the ones' place of the second number (3). The result is 1 * 3 = 3. Place the answer below the line and to the left of the 6.

  
  312
    3
   36

• Multiply the digit in the hundreds' place column (3) by the second number (3). The result is 3 * 3 = 9. Place the 9 below the line in the hundreds' place column.

  
  312
    3
  936