(Topic 5 Task 3) Multiplication Tips and Tricks

Overview

This lecture reviews tips and tricks to make multiplication easier, focusing on shortcuts, patterns, and memory aids.

Multiplying by 2: Double the number (e.g., 2×9 = 18).
Multiplying by 4: Double the number twice (e.g., 4×9: 9→18→36).
Multiplying by 5: Halve the number, then multiply by 10 (e.g., 5×6: 3×10=30).
Multiplying by 5 (alternate): Multiply by 10, then halve (e.g., 5×9: 9×10=90; 90÷2=45).
5 multiples alternate between ending in 0 and 5.
Multiplying by 6 (even numbers): Product ends in the same digit as the number (e.g., 6×2=12; both end in 2).
56 = 7×8; remember with "5, 6, 7, 8."
Multiplying by 8: Triple doubles (double three times, e.g., 8×6: 6→12→24→48).
Multiplying by 9: Multiply by 10, then subtract the number (e.g., 9×6: 60–6=54).
For 9s: Ones digit decreases by 1; tens digit increases by 1 (e.g., 9×5=45).
Fingers trick for 9s: Hold down the finger for the number being multiplied and count the left and right fingers.
Multiplying by 10: Add a 0 (e.g., 10×2=20).
Multiplying by 11 (up to 9): Repeat the digit (e.g., 11×4=44).
Multiplying by 11 (10–18): Insert the sum of the digits between them (e.g., 11×15: 1_(1+5)_5=165).
Multiplying by 12: Multiply by 10 and add double (e.g., 12×4=40+8=48).
Multiplying by 15: Multiply by 10, then add half again (e.g., 15×4=40+20=60).
Multiplying by 20: Multiply by 10, then double (e.g., 20×4=40+40=80).

Memorize squares (e.g., 6×6=36, 9×9=81) to speed up multiplication.
Trick: For numbers two apart (e.g., 7 and 5), square the middle number and subtract one for the product (7×5 = 6×6–1 = 35).