Multiplication with large numbers
You learn to multiply big numbers together, with one row for each digit.
How to work it out
- Row 1 multiplies by ones, row 2 starts with one 0, row 3 with two 0s.
- Add all rows together.
124
×23
372
+2480
2852
A worked example
124 × 23
- Row 1: 3 × 124 = 372.
- Row 2 starts with a 0: 20 × 124 = 2480.
- Add the rows: 372 + 2480 = 2852.
More examples
465 × 32
- Row 1: 2 × 465 = 930.
- Row 2 starts with a 0: 30 × 465 = 13,950.
- Add the rows: 930 + 13,950 = 14,880.
465
×32
930
+13950
14880
213 × 121
- Row 1: 1 × 213 = 213.
- Row 2 starts with a 0: 20 × 213 = 4260.
- Row 3 starts with two zeros: 100 × 213 = 21,300.
- Add the rows: 213 + 4260 + 21,300 = 25,773.
213
×121
213
4260
+21300
25773
Watch out
You start the rows without zeros, so they end up too far to the right.
Keep track of which digit you are on: the second gets one 0, the third gets two zeros.
Tips
- Row 2 starts with one 0, row 3 with two zeros.
- Estimate first: 124 × 23 is about 120 × 20, so around 2400.
Why it works
Each digit of the bottom number gets its own row. The tens digit is worth ten times as much, so that row starts with a 0. The hundreds digit is worth a hundred times as much, so that row starts with two zeros. That way everything is in the right place.