Laws that make multiplying easier
Example 1: mental strategies●Work out (a) $4 \times 17 \times 25$ and (b) $7 \times 48$.
(a) $4 \times 25 = 100$, so $100 \times 17 = 1700$.
Reorder to make a nice pair.
(b) $7 \times 48 = 7 \times 50 - 7 \times 2 = 350 - 14 = 336$.
Distributive law.
Long multiplication
Find $347 \times 26$.
$347 \times 20 = 6940$
The 2 in 26 is worth 20.
Add: $2082 + 6940 = 9022$.
Estimate first: $350 \times 25 \approx 8750$, so $9022$ is reasonable.
Distributive law
$a(b + c) = ab + ac$
$25 imes 4$
$100$, a handy pair to look for
$ imes 1000$
Move digits three places left (add three zeros).
Find $4 \times 17 \times 25$.
Find $7 \times 48$.
Find $347 \times 26$.
Find $450 \times 100$.
Which law says $6 \times (10 + 3) = 60 + 18$?
The $6$ is distributed over the sum.
Find $9 \times 120$.
$9 \times 12 = 108$, then $\times 10$.
The best estimate of $398 \times 21$ is:
A stadium has $56$ rows of $36$ seats. How many seats are there?
New numbers every time. Keep going until you get five in a row.
Coaching Centre