

For more practice,

The Wonder Number Lessons
Lesson No. 6: Subtracton, DoubleDigit Understanding place value in double digit subtraction is a difficult concept. This concept becomes much clearer and easier to visualize when using the Wonder Number Board. The first example of double digit subtraction is 68  23. (Remind your students that ones are subtracted horizontally, from right to left on the board, and sets of tens are subtracted vertically, from bottom to top on the board). Begin by placing a viewer on 68. Explain that subtraction means taking one number away from another number to find the difference. First subtract the three ones by counting aloud and moving backward from 68 by placing red chips on 67, 66, and 65. Now subtract the two sets of ten by counting aloud and moving up, placing yellow chips on 55 and 45 (see fig 29). The student will discover the answer is 45. To selfcheck, lift up the last yellow chip and verify the answer. The second double digit subtraction problem is 78  56. Begin by placing a viewer on 78. First subtract the six ones by counting aloud and move backward from 78 by placing yellow chips on 77, 76, 75, 74, 73, and 72. Now subtract the five sets of ten by counting aloud and moving up, placing blue chips on 62, 52, 42, 32, and 22 (see fig 30). The student will discover the answer is 22. To selfcheck, lift up the last blue chip and verify the answer. The third example of subtraction presents a new challenge because it involves moving up the board while subtracting ones. The problem is 66  27. Begin by placing a viewer on 66. First subtract the seven ones by counting backward from 66 and placing yellow chips on 65, 64, 63, 62, 61, 60, and 59. Now subtract the two sets of ten by counting up and placing blue chips on 49 and 39 (see fig 31). The student will discover the answer is 39. To selfcheck, lift up the last blue chip and verify the answer.

