Decimals Operations Subtraction (lesson 2)

Age

9-12

Materials

• Decimal board
• Decimal beads and disks
• Three sets of decimal numeral cards
• Cardboard strip as long as the decimal board
• Prepared problems
• Paper and pencil

• Optional/Alternative: Montessori bead bars

Preparation

Students should have had the Introductory Decimal Lessons 1-9 and Addition with Decimals lesson 1.

Presentation

• Teacher can choose to use bead bars for this lesson to help make an impression for the difference in values.

• 1. Select the first problem: 6.4 - 2.8
• 2. Form the minuend and subtrahend with cards, placing the cards to the left of the board with the cardboard strip underneath (emphasizing the importance of placing the decimal disks in the proper place values).
• 3. Place the beads and disks for the minuend on the board. Beginning with the tenths, remove the amount in the subtrahend, exchanging a unit for ten more tenths. Place the quantity taken away on the table beside the board, so it can be used to check work at the end.
• 4. Form the remainder with cards, and place them under the other cards. Have the child record the problem in his notebook.

NOTE: Provide problems which involve a great deal of exchanging, e.g., 3.000 - 0.123 and 1.000000- 0.999999

• Again, most students will work with this material for a very short time, because they will be able to internalize the procedure and proceed abstractly.

Control Of Error

Teacher guided; lining up the disks in the proper decimal place values.

Points Of Interest

Participating with the new knowledge helps move the concrete experiences toward abstract thinking

Purpose

• To make a visual, auditory and kinesthetic impression upon the student on how to perform operations with decimals.

Variation

Alternative: Use bead bars instead of beads and disks.