Multiplication Tips

Multiplying 2-digit numbers (same 1st digit)

• Fifth grader Mark Labib writes: My trick is about multiplying any 2 digit number by any other 2 digit number as long as they have the same tens digit. For example,
  

  23 × 26   OR   42 × 47   OR    51 × 59
  

• As it gets harder with the larger numbers, I will start with multiplying two numbers in the twenties. For example,
  

  23 × 27
  

Step 1. Add the 3 (from the 23) to the 27: 3 + 27 = 30

Step 2. Multiply the result by 10: 30 × 10 = 300

Step 3. Double that result: 2 × 300 = 600

Step 4. Multiply 3 (from the 23) and 7 (from the 27): 3 × 7 = 21

Step 5. Add these last two products: 600 + 21 = 621

• For two numbers in the thirties, when you come to step 3, triple the result; for the forties, quadruple it (just by doubling the number twice); for the fifties, multiply by 5 (using the tips for multiplication by 5: divide the number by 2 and multiplying it by ten); and so on.