3
Rows, columns and total stars
Discover Mathematics P3: Term 1, Lessons 11–20
GoalDescribe an array with rows and columns and find the total with repeated addition.
New words
array · things in equal rows and equal columnsrow · a line across, from side to sidecolumn · a line down, from top to bottom
Explanation
An array puts things in neat rows and columns. Rows go across and columns go down. In this array of stars there are 4 rows and 6 stars in each row. To find the total, add the rows: 6 + 6 + 6 + 6 = 24, or skip count by 6: 6, 12, 18, 24. The same array has 6 columns with 4 stars in each column. Groups drawn in circles can be anywhere, but in an array the groups line up, so we can see both rows and columns. First count the rows, then count how many are in one row.
Worked examples
This array has 3 rows with 5 stars in each row. Add the rows: 5 + 5 + 5 = 15 stars. It also has 5 columns with 3 stars in each column.
Twelve dots can be put in three circles of 4, or in an array of 3 rows with 4 dots in each row. The total is the same: 4 + 4 + 4 = 12. The array also shows 4 columns with 3 dots in each column.
Karim makes an array with 7 rows and 2 dots in each row. He adds 2 + 2 + 2 + 2 + 2 + 2 + 2 = 14 dots. Skip counting by 2 gives 2, 4, 6, 8, 10, 12, 14.
Class activity
Arrays around us. Look for arrays at home and in the street: an egg tray, tiles on a floor, windows of a building, seats on a bus. For each one count the rows, count how many are in a row, and find the total by repeated addition.
Practice
1
How many rows are there, and how many stars in each row? How many stars in all?
4 rows of 5 stars. 5 + 5 + 5 + 5 = 20 stars.
2
An array has 6 rows with 3 stars in each row. Write the repeated addition and the total.
3 + 3 + 3 + 3 + 3 + 3 = 18 stars.
3
A classroom has 5 rows of desks with 6 desks in each row. How many desks are there?
6 + 6 + 6 + 6 + 6 = 30 desks.
4
Is this an array? Count the rows and the columns and find the total.
Yes. It has 3 rows of 6 and 6 columns of 3. 6 + 6 + 6 = 18 dots.
5
Draw an array with 4 rows and 4 stars in each row. How many stars are there?
4 + 4 + 4 + 4 = 16 stars.