Multiply by one digit

In Primary 2, you learned to add with renaming. Here are 10 ones. 10 ones make 1 ten. Today, we will multiply by a one-digit number, one place at a time, starting with the ones. A stall packs one hundred and twenty-six boxes of curry puffs. Each box has 3 curry puffs. How many curry puffs are there altogether? Each box is a group of 3 curry puffs. So there are one hundred and twenty-six groups of 3. We multiply one hundred and twenty-six by 3. Hundreds, tens and ones each have their own column. Write one hundred and twenty-six on top. Write 3 under the ones, with a times sign. Start with the ones. 6 ones times 3 is 18 ones. 18 ones is 1 ten and 8 ones. Write 8 in the ones. Write the 1 ten on top of the tens. Now the tens. 2 tens times 3 is 6 tens. Multiply first. Then add the 1 ten on top. 6 tens plus 1 ten is 7 tens. Now the hundreds. 1 hundred times 3 is 3 hundreds. There is nothing on top, so write 3 in the hundreds. So there are three hundred and seventy-eight curry puffs altogether.

A stall packs two hundred and forty-seven bags of fish balls. Each bag has 4 fish balls. How many fish balls are there altogether? Try it first. Pause if you like. 7 ones times 4 is 28 ones. 28 ones is 2 tens and 8 ones. Write the 2 tens on top of the tens. 4 tens times 4 is 16 tens. Then add the 2 tens on top. 16 tens plus 2 tens is 18 tens. 18 tens is 1 hundred and 8 tens. Write the 1 hundred on top of the hundreds. 2 hundreds times 4 is 8 hundreds. Then add the 1 hundred on top. 8 hundreds plus 1 hundred is 9 hundreds. So there are nine hundred and eighty-eight fish balls altogether.

A hawker centre has three hundred and five tables. There are 6 stools at each table. How many stools are there altogether? Try it first. Pause if you like. 5 ones times 6 is 30 ones. 30 ones is 3 tens and 0 ones. Write the 3 tens on top of the tens. 0 tens times 6 is 0 tens. We must still add the 3 tens on top. 0 tens plus 3 tens is 3 tens. 3 hundreds times 6 is 18 hundreds. 18 hundreds is 1 thousand and 8 hundreds. There are no thousands to multiply, so write 1 in the thousands. So there are one thousand, eight hundred and thirty stools altogether. Multiply one place at a time, starting with the ones. Multiply first, then add the number on top.