Step 1

Multiply the top number by the ones (21 x 2).

   21
x 42
   42

 

 

 

Step 2

Place a zero in the ones place, below the product of Step 1.

  21
x 42
  42
0

Step 3

Multiply the top number by the tens (21 x 4) and place it next to the zero from Step 2.

  21
x 42
  42
840

 

 

 

Step 4

Add the bottom two numbers (42 + 840).

  21
x 42
  42
840
882