The method from the last lesson works well for any multiplication of a 2-digit number by a 1-digit number, including multiplication by ×9.
In this short lesson, I’ll show you an alternative way to multiply a 2-digit number by ×9—for example, 46 × 9.
Multiplication by ×9 is almost multiplication by ×10, and multiplication by ×10 is very easy (just add a zero). So we reinterpret the question as a simple subtraction:
Example A: 46 × 9 → 460 – 46 = 414
Note that the first number (here it’s 460) always ends in zero—because it is a multiple of 10. This makes the subtraction easier.
Example B: 67 × 9 → 670 – 67 = 603
Example C: 38 × 9 → 380 – 38 = 342
Example D: 83 × 9 → 830 – 83 = 747
Example E: 77 × 9 → 770 – 77 = 693
Note that examples A, B and C are easier than examples D and E because the hundreds digit doesn’t change. Let’s take example C (we stay in the 300s) and compare with example D (we don’t stay in the 800s but drop into the 700s).
If the units-digit (8 in 38, or 3 in 83) is larger than the tens digit (examples A, B and C) then the subtraction is easy. If it is smaller (example D) then it is more challenging.
I recommend you try this method and see whether you prefer it to the standard multiplication method from the previous lesson.
The next section is about spoken subtractions, which will also help you with this alternative method.