One row is ten
A hundred square has 10 rows, and each row has 10 squares. Count along the first row in ones, touching each square once: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. One full row is ten.
The first row is shaded: 10 squares.
Every row adds another ten
The second row runs from 11 to 20, and the third row from 21 to 30. Each new row adds another ten, so three full rows are 10, 20, 30.
So once you know the rows are full, count the rows instead of the squares: 10, 20, 30, 40, 50, 60, 70. Seventy squares counted in ones takes seventy numbers. Counted in tens it takes seven, one for each row.
The names of the tens tell you how many tens there are. Forty is 4 tens, sixty is 6 tens and seventy is 7 tens. Twenty, thirty and fifty are 2, 3 and 5 tens, even though their names sound a little different.
Seven full rows are shaded: 10, 20, 30, 40, 50, 60, 70.
Each step along this line is ten more, the same as one more row of the hundred square.
3 jumps of 10 = 30
Count on to 100 in jumps of 10
Each jump of 10 lands at the end of the next row down: 10, 20, 30, and on to 100 at the bottom.
Ten tens make a hundred
Keep counting in tens: 80, 90, and the tenth row fills the square. Ten rows of ten is one hundred, written 100, and 10 × 10 = 100.
All 10 rows are shaded: ten tens is 100.
Moving around the hundred square
Counting in ones, each row ends on a ten, and the next number starts a new row. After 39 comes 40, not "thirty-ten", because 10 more ones have made another ten.
The rows make it quick to find ten more or ten less. 37 is in the row that runs from 31 to 40. The square just to its right is 1 more, 38. The square just below it is a whole row further on, which is 10 more, 47. The square just above it is 10 less, 27.
Worked example: Ten More, Ten Less and a Hundred More
Question Devi puts a counter on 64 on a hundred chart. Each row of the chart has ten numbers. (a) What number is just below 64, and what number is just above it? (b) What number is 100 more than 64?
1.Find 64 on the chart. It is in the row that goes from 61 to 70.
Here are three rows of the hundred chart. 64 is in the row that goes from 61 to 70. 2.One row down is 10 more. The tens digit goes up by 1 and the ones digit stays the same: 64 + 10 = 74.
One row down is 10 more. Only the tens digit changes: 64 + 10 = 74. 3.One row up is 10 less. The tens digit goes down by 1: 64 − 10 = 54.
One row up is 10 less: 64 − 10 = 54. 4.(a) The number just below 64 is 74, and the number just above it is 54.
(a) 74 is just below 64, and 54 is just above it. 5.(b) 100 more is 1 more hundred. The tens and the ones stay the same: 64 + 100 = 164.
(b) 100 more is 1 more hundred. The tens and the ones stay the same: 64 + 100 = 164.
Answer: (a) 74 is below and 54 is above; (b) 164
Common mistakes
- Saying that the number below 64 is 65. The number 65 is next to 64 in the same row, which is 1 more. One row down is 10 more.
- Writing 1064 for 100 more than 64. One more hundred puts a 1 in the hundreds place, so the number is 1 hundred, 6 tens and 4 ones, which is 164.