Egypt Primary 3 Mathematics, Week 29: Threes help with fours

Tens facts, threes and fours, and year review

Week 29 · Tens, threes and fours, and the year in review · 3 lessons

Threes help with fours

Discover Mathematics P3: Term 2, Lessons 111–120

GoalUse threes facts to find fours facts by adding one more group, or by doubling twice.

New words

one more group · add one more group of the same sizedouble twice · double a number, then double the answerarray · things set in equal rows and columns

Explanation

A fours fact is a threes fact plus one more group. For 4 × 7, we know 3 × 7 = 21. One more group of 7 gives 21 + 7 = 28, so 4 × 7 = 28. The same idea works with any number: 4 × n = 3 × n + n. Another way is to double twice, because 4 is 2 and 2. For 4 × 7, double 7 to get 14, and double 14 to get 28. Both ways give the same answer. In an array, 4 rows of 7 is 3 rows of 7 and one more row of 7. These tricks work for big numbers too: 4 × 12 = 3 × 12 + 12 = 36 + 12 = 48.

Number	3 × number	4 × number
3	9	12
4	12	16
5	15	20
6	18	24
7	21	28
8	24	32

Worked examples

The array for 4 × 7 has 4 rows of 7. Three rows make 3 × 7 = 21, and the fourth row adds 7 more: 21 + 7 = 28.

Way	Work	Answer
Threes plus one	27 + 9	36
Double twice	18 + 18	36

Find 4 × 9. Using threes: 3 × 9 = 27 and 27 + 9 = 36. Doubling twice: 9 + 9 = 18 and 18 + 18 = 36. Both give 36.

Find 4 × 12 with the model. Three groups of 12 are 36, and one more group of 12 gives 36 + 12 = 48.

Class activity

Threes to fours. Make a two-column chart with the numbers 3 to 12 in the first column. Write the threes fact in the second column and the fours fact in the third. For each row use the rule four = three plus one more group. Colour the facts you knew by heart and practise the others.

Practice

Find 4 × 6 using 3 × 6.

Use the model to find 4 × 11.

Find 4 × 8 by doubling twice.

A table has 4 legs. How many legs do 9 tables have?

Salma says 4 × 7 = 27. Use threes to check her answer.