Multiplying

Know the times table through 12 cold; everything else is built from it.

×123456789101112
1123456789101112
224681012141618202224
3369121518212427303336
44812162024283236404448
551015202530354045505560
661218243036424854606672
771421283542495663707784
881624324048566472808896
9918273645546372819099108
10102030405060708090100110120
11112233445566778899110121132
121224364860728496108120132144

The highlighted diagonal is the perfect squares. The table is symmetric: 7 × 8 is the same as 8 × 7, so there are only half as many facts to learn as it looks.

Bigger numbers: break one apart

Split one number into tens and ones, multiply each part, and add: 46 × 23 = 46 × 20 + 46 × 3 = 920 + 138 = 1,058. This is exactly what the written method does, one row per digit.

Zeros

Multiply the nonzero parts, then tack on all the zeros: 300 × 40 = 12 followed by three zeros = 12,000. Anything times 0 is 0; anything times 1 is itself.

AFOQT questions on this: Order of operations · Unit price