Multiply by 4, 8, or 16

Here's a fast trick for multiplying numbers by 4, 8, or 16 using only doubling!

Just double the number however many times you need:
• 4 = 2 × 2 (double twice)
• 8 = 2 × 2 × 2 (double three times)
• 16 = 2 × 2 × 2 × 2 (double four times)

Let's try multiplying by 4:

Step 1: Double the number

Step 2: Double again

Thus 23 × 4 = 92

Now let's multiply by 8:

Step 1: Double → 104

Step 2: Double again → 208

Step 3: Double one more time → 416

Thus 52 × 8 = 416

Notice we can use this method on any power of 2, 4, 8, 16 and so on, just keep doubling.