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Multiplying Decimals

The Hook

Counting the hops.

What is it?

Multiplying numbers with dots.

Time Travel (History)

Essential for calculating tax and discounts.

Cheat Code

Ignore the decimals, multiply normally, then put the dot back by counting total decimal places.

Practice This Now!

The Field Guide

In Plain English: Pretend the decimals don't exist. Multiply 0.5 x 0.3 as if it was 5 x 3 (which is 15). Then count how many numbers were behind decimals (2). Move the dot 2 spots left. Answer: 0.15.

In The Real World: Calculating 8.25% sales tax on a $19.99 item.

How To Do It

  1. Ignore dots.
  2. Multiply.
  3. Count total decimal places in original problem.
  4. Move dot in answer that many steps left.
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Booby Trap!

Lining up the dots like addition. DO NOT line up the dots! Line up the right wall.

Real World Challenge
"Gas costs $3.50 a gallon. You pump 10.5 gallons."
The Logic: 350 x 105 = 36750. Total decimals = 3 (2 from price, 1 from gas). Move dot 3 times -> $36.75.

Training Drills

Counting Hops
0.2 (1 hop) x0.3 (1 hop) ---- 0.06 (2 hops)
2x3=6. Move dot twice.