1
Hand lengths on a dot plot
Discover Mathematics P3: Term 1, Lessons 1–10
GoalMeasure lengths in centimetres with a ruler, record them in a table and show them on a dot plot.
New words
ruler · a tool with marks for measuring lengthcentimetre (cm) · a small unit of length, about the width of a fingermeasurement · a number with a unit that tells how long something is
Explanation
To measure with a ruler, put the zero mark at one end of the object and keep the ruler straight along it. Read the number at the other end and write it with its unit, for example 8 cm. If the end is between two marks, choose the nearest whole centimetre. To order lengths, write them from the shortest to the longest. A dot plot of lengths has one dot above each length on a number line, and the number line must show the unit.
Worked examples
Youssef lays a pencil along a ruler. One end is at 0 and the other end is at 13. The pencil is 13 cm long.
Salma has five pieces of string: A is 9 cm, B is 14 cm, C is 6 cm, D is 12 cm and E is 8 cm. From shortest to longest they are C 6 cm, E 8 cm, A 9 cm, D 12 cm and B 14 cm.
Ten students measured their hands from the wrist to the tip of the middle finger. The dot plot shows 1 hand of 11 cm, 3 hands of 12 cm, 2 hands of 13 cm, 3 hands of 14 cm and 1 hand of 15 cm. The two most common lengths are 12 cm and 14 cm.
Class activity
Measure hands. Use a ruler, or a strip of paper marked in centimetres. Measure the length of the hand from the wrist to the tip of the middle finger for 6 people in your family or class. Write the lengths in a table, put them in order, and make a dot plot with a title and a unit on the number line.
Practice
1
How long is the line?
7 cm.
2
The crayon does not start at 0. It starts at 3 and ends at 11. How long is it?
8 cm, because 11 − 3 = 8.
3
Four ribbons are 18 cm, 7 cm, 25 cm and 12 cm long. Put them in order, shortest first.
7 cm, 12 cm, 18 cm, 25 cm.
4
This plot shows the hand lengths of 8 other students. How many of them have a hand of 12 cm or longer?
5 students: 3 + 1 + 1 = 5.
5
Marwan puts a card on a ruler. One end is at 1 and the other end is at 11. He says the card is 11 cm long. Is he right?
No. The card is 11 − 1 = 10 cm long. He should start at 0.