Tuesday
Tuesday: The standard algorithm
Lucy the Ladybug is helping Bennie stack crates of apples.
"Each crate holds 46 apples, and we have 27 crates," says Bennie.
Lucy looks at the crates. "Yesterday we broke numbers into four pieces. Is there a shorter way?"
"Yes," says Bennie. "The standard algorithm uses the same idea in two neat rows."
"First we multiply 46 by the 7 ones. Then we multiply 46 by the 2 tens."
Lucy smiles. "Two rows feels calm and tidy. Let us try it!"
Learn it: two rows, then add
Write 46 on top and 27 below it, lined up by place.
Row 1: multiply 46 by 7 ones. 46 × 7 = 322.
Row 2: multiply 46 by 2 tens. 46 × 20 = 920.
Write a 0 in the ones place of row 2, because we are multiplying by tens.
Add the rows: 322 + 920 = 1,242.
When the ones make more than 9, regroup. In 46 × 7, 6 × 7 = 42.
Write the 2 and carry 4 tens. Then 4 tens × 7 = 28 tens, plus 4 tens is 32 tens.
- Think about the two rows.
- Tap the steps in order.
- Read the question.
- Tap your answer.
- Read the question.
- Tap your answer.
- Read the question.
- Tap your answer.
| Sentence | True or false? |
|---|---|
| 46 × 27 = 1,242 | ? |
| 46 × 27 = 322 + 92 | ? |
| 58 × 34 = 1,972 | ? |
| Row 2 needs a 0 in the ones place when we multiply by tens. | ? |
Imagine 23 cartons just like it.
Multiply 12 × 23 on paper with two rows.
Check by adding 12 three times, then 12 twenty times.
Calm and tidy! Tomorrow we multiply even bigger numbers.