Explain that with standard algorithm multiplication you start by putting the numbers one under the other. You put the largest number on top. The T, H, T, O above the table refer to the thousands, hundreds, tens, and ones. Show that above each column there is a box for regrouping. In these boxes you can write down regrouped numbers to add them up later with the other outcomes. Explain that you start with the lower number on the right in the ones column. You multiply this number with the higher number from the ones column (5 × 3 = 15). From this 15 you write the 5 under the line in the ones column. You can write the 1 (10) in the regrouping box of the tens. Now multiply the lower number from the ones column with the higher number from the tens column (5 × 2 =10). Emphasize that you shouldn't forget to add the 1 from the regrouping box of the tens to that number (10 + 1 = 11). From this 11 you write the 1 under the line in the tens column. You can write the other 1 (100) in the regrouping box of the hundreds. Next you multiply the lower number in the ones column with the higher number from the hundreds column (5 × 1 = 5). Also, don't forget to add the 1 from the regrouping box of the hundreds to this (5 +1 = 6). Write the 6 under the line in the hundreds column. The answer is 615. Practice two problems together with the students that must be calculated using the standard algorithm. The students may choose to use a sheet of scrap paper. The students solve the next three problems on their own.
Check whether the students can multiply using the standard algorithm with a number to 1,000 by asking the following question:
- What steps do you take to solve the problem 251 × 4 using the standard algorithm?