Mental Math Grade 6 Teacher's Guide

C. Mental Computation—Subtraction
• Back Down Through 10/100/1000 (Review)
It is reasonable to expect most students to mentally
keep track of no more than two combinations,
especially if there is trading involved.
This strategy extends one of the strategies students learned in Grade 3 for
fact learning. It involves subtracting a part of the subtrahend to get to the
nearest ten or hundred, or thousand and then subtracting the rest of the
subtrahend. It was introduced in grade 3 for fact learning, extended to
numbers in the 10’s and 100’s in grade 4, and to numbers in the 1000’s in
grade 5.
Examples
a) For 15 – 8, think, “15 subtract 5 (one part of the 8) is 10, and 10
subtract 3 (the other part of the 8) is 7.”
b) For 74 – 6, think, “74 subtract 4 (one part of the 6) is 70 and 70
subtract 2 (the other part of the 6) is 68.”
c) For 530 – 70, think, “530 subtract 30 (one part of the 70) is 500 and
500 subtract 40 (the other part of the 70) is 460.”
d) For 8600 – 700, think, “8600 subtract 600 (one part of the 700) is
8000 and 8000 subtract 100 (the rest of the 700) is 7900.”
Practice Items
74 – 7 = 97 – 8 = 53 – 5 =
420 – 60 = 340 -70 = 630 – 60 =
540 – 70 = 760 – 70 = 320 – 50 =
9200 – 500 = 4700 - 800 = 6100 – 300 =
7500 – 700 = 8 00 – 600 = 4200 – 800 =
9500 – 600 = 3400 – 700 = 2300 – 600 =
Add your own practice items
32 Mental Math – Grade 6