Multiplying Orally

Let's find the products orally using the zero trick. For problem (a), the product of $44 \times 100$ is . In problem (d), multiplying $9 \times 40$ gives us . Moving to (g), calculating $9 \times 8,000$ results in . For problem (h), the product of $31 \times 200$ is . For (n), multiplying $9 \times 80$ yields . Finally for (p), the product of $459 \times 1,000$ is . Using this rule makes mental multiplication fast and easy.

We can practice finding both missing products and missing factors by counting zeros. In a forward problem like $75 \times 1,000$, we just append the zeros to get . For a reversed problem like $25 \times \_\_\_ = 2,500$, we look at the extra zeros to find the missing factor is . Similarly, if $\_\_\_ \times 1,000 = 68,000$, the missing starting number before the zeros were added must be . Finally, for the equation $33 \times \_\_\_ = 33,000$, counting the three extra zeros tells us the missing factor is . This pattern works reliably for all round numbers.