Students can tackle mental calculation of the differences of two 2-digit numbers in this lesson by reflecting on the discussion of mentally calculating the sums of two 2-digit numbers in the previous day's lesson. Students will be thinking about ways to mentally calculate by decomposing numbers based on different properties of operations and using approximate numbers. It is hoped that students will be able to take advantage of their
learning in the previous lesson. Therefore, in today's lesson, the emphasis is placed on representing own ideas using equations, expressions, diagrams, and words, and organizing their mental calculation processes clearly. For this purpose, it is necessary that students must interpret other's ideas and compare and contrast with their own ideas. During the whole class
discussion, the activity of interpreting equations and expressions and representing ideas expressed in words using equations and expressions. Then, by having students reflect on their own ideas in light of other's ideas, we hope to help students identify more efficient(faster and simpler) ways of mental calculation.

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The teacher shows "□-28" and three numbers 68, 53 and 89, and asks the student which number they would like to put into □. After sharing we can easily calculate 68-28 and 89-28 by mental arithmetic but not 53-28, he poses a problem.
(7:09)

Solving Problem

Students independently work on the problem that the teacher posed. While each student works on the problem, the teacher observes how each student is solving problem, consider in what sequence the various solution strategies may be shared and discussed.
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Sharing: Borrowing

After the individual work by the students, the teacher asks students to share what they have done. A student who use borrowing answer and 2 more students explain the similar ways.
(3:21)

Sharing: Seperating

The teacher asks for different ways. A student expresses her way, that is, 53-3=50, 50-5=45 and 45-20=25. Another student finds this method isn't using 28. Through the discussion, they find that 28 is separated into 3, 5 and 20.
(4:28)

Discussion: Does this makes sense?

One student who did 8-3 instead of 3-8 expresses his way. After sharing this method leads to the right answer for 53-28, the teacher suggest 42-17. Students find the method doesn't work for it.
(5:46)

Sharing: Character of Subtraction

One student expresses her way, that is, add 2 on 28 to make the calculation simpler and also on 53. Students discuss about this way and find it depends on the rule of subtraction through the discussion.
(4:39)