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Maths
Summary
There are lots of different ways to multiply a two-digit number by a one-digit number, using both mental methods and written 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.
Numbers can be broken down into simpler calculations.
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.
1. | Split the two-digit number into tens and ones. |
2. | Multiply the tens and ones by the one digit number separately. |
3. | Add the multiplied parts together. |
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.
$180+12=\underline{192}$
In some cases it may be simpler to multiply by a larger number close to the two-digit number and then take away lots of the one-digit number.
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.
$80-4=\underline{76}$
FAQs
Question: What is a mental method?
Answer: 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.
Question: How do I multiply a one-digit number by 19?
Answer: Instead, multiply by 20 and take one lot of the one-digit number away.
Question: How do I multiply a two-digit number by a one-digit number in my head?
Answer: You can multiply a two-digit number by a one-digit number in your head using partitioning: 1. Split the two digit number into tens and ones. 2. Multiply the tens and ones by the one digit number separately. 3. Add the multiplied parts together.
Theory
Exercises
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