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Lesson 25 Section 2 ## SUBTRACTING MIXED NUMBERS
But consider the following, in which the fractions are reversed:
To see how to deal with it, consider the following: We cannot take 40 minutes from 10 minutes -- we need more minutes. To get them, we will break off 1 of the 7 hours, and 60 minutes + 10 minutes = 70 minutes: 2 hours from 6 hours is 4 hours. 40 minutes from 70 minutes is 30 minutes.
5 feet from 7 feet is 2 feet. 11 inches from 12 inches is 1 inch. We can now return to our problem:
We need more fifths. Where will we get them? From 8. We will break off 1 from 8, and decompose it into . (Lesson 21, Example 4.) We will then Then:
* Actually, the simplest way to do this problem is mentally, by rounding off 1 to 2. (Compare Lesson 7.) That is, add to both numbers. 8 − 1 = 8 − 2 = 6.
Now the mystery, if any, is: How does that numerator get to be 9? 9 is the
7 is the sum of denominator plus numerator: 5 + 2.
Again, the simplest way to do this is mentally by rounding off 1 to 2. Add . (Lesson 21.) The problem then becomes 6 − 1 = 6 − 2 = 4.
Compare Lesson 7, Example 2.
We cannot take from ; therefore, on taking —1—from 6, the fraction becomes 4-fourths + 2 -fourths = 6-fourths:
Mentally:
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