Fast Multiplication Techniques

Multiplication appears constantly in QR: calculating total costs (quantity × price), finding areas (length × width), scaling recipes, and more. The on-screen calculator handles multiplication, but for many common multiplications, mental techniques are significantly faster. This lesson covers the most useful shortcuts.

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Multiplying by 5, 50, and 500

These all exploit the relationship between 5 and 10.

Multiply by 5: ×10 then ÷2

Since 5 = 10 ÷ 2:

Calculation	×10	÷2	Answer
48 × 5	480	240	240
73 × 5	730	365	365
156 × 5	1,560	780	780

Multiply by 50: ×100 then ÷2

Since 50 = 100 ÷ 2:

Calculation	×100	÷2	Answer
36 × 50	3,600	1,800	1,800
84 × 50	8,400	4,200	4,200
125 × 50	12,500	6,250	6,250

Multiply by 500: ×1,000 then ÷2

Since 500 = 1,000 ÷ 2: