60
Reading and writing 21–100
Textbook Part 2, pp. 40–41
GoalRead and write two-digit numbers; split a number into tens and ones; meet the history of our digits.
New words
Arabic (Hindu-Arabic) numerals · arab raqamlaritens and ones · xona qo‘shiluvchilarireverse order · teskari tartib
Explanation
The digits we use are called Arabic numerals. People learned them from books written in Arabic; one of the first books on this mathematics was written by the great scholar from Khorezm, Muhammad ibn Musa al-Khwarizmi. Any two-digit number can be written as tens plus ones: 47 = 40 + 7.
Worked examples
Ancient Egyptians wrote 1 as a “stick” and 10 as a “heel bone”. 35 = 3 heel bones and 5 sticks.
The teacher was given tickets in fives: 5 + 5 + 5 + 5 + 5 + 5 + 5 + 5 = 40.
Class activity
“Egyptian writing”: children write their age, the number of pupils in the class and one other number in ancient Egyptian signs; a partner reads them back.
Practice
1
Write as tens plus ones: 68
60 + 8
2
Write in digits: ninety-four
94
3
Each row of an album holds 10 coins. 5 rows are full. How many coins?
50
4
Count backwards from 75 to 68.
75, 74, 73, 72, 71, 70, 69, 68