Multiplying a 2digit number by a 1digit number
In a nutshell
There are lots of different ways to multiply a twodigit number by a onedigit number, using both mental methods and written methods.
Mental methods
Any method that can be done in your head without writing down working out is a mental method. Different mental methods can be used depending on the question.
Breaking down
Numbers can be broken down into simpler calculations.
Example 1
What is $5\times40$?
Break $40$ down.
$40=4\times10$
Work out new calculation.
$\begin {aligned} 5\times4\times10&=20\times10\\&=\underline{200}\end {aligned}$
Tip: Remember, to multiply by any multiple of ten, first times by the number in the tens column, then times by ten.
Partitioning
Procedure
1.
 Split the twodigit number into tens and ones.

2.
 Multiply the tens and ones by the one digit number separately.

3.
 Add the multiplied parts together.

Example 2
What is $64\times3$?
Split $64$ into tens and ones.
$64=60+4$
Multiply both $60$ and $4$ by $3$ separately.
$\begin {aligned}60\times3&=10\times6\times3\\&=10\times18\\&=180\\4\times3&=12\end{aligned}$
Add the parts together.
$180+12=\underline{192}$
Taking away
In some cases it may be simpler to multiply by a larger number close to the twodigit number and then take away lots of the onedigit number.
Example 3
What is $19\times4$?
Instead, work out $20\times4$.
$\begin {aligned}10\times2\times4&=10\times8\\&=80\end {aligned}$
This is equivalent to $20$ lots of four. To work out $19$ lots of four, take one lot of four away.
$804=\underline{76}$