Double and Half Trick

This trick involves multiplying by a clever version of 1, specifically $\frac{2}{2}$. You double one of the numbers in a multiplication problem and halve the other. This is especially useful when one number ends in 5 or is an even number in the teens.

• $15 \times 78 = (15 \times 2) \times \frac{78}{2} = 30 \times 39 = 1170$
• $35 \times 42 = 70 \times 21 = 1470$
• $18 \times 52 = 9 \times 104 = 936$