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Grade 5 Module 3 Lesson 14 Problem Set

 
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Grade 5 Module 3 Lesson 14 Problem Set
by Gloria Wilson - Wednesday, January 18, 2017, 7:07 PM
 

I am confused as to how to teach problem 1. C.  I understand commutative and associative properties, but when the problem reads 4 3/7 - 3/4 -2 1/4 -3/7 = ?  The answer key moves the numbers but then adds 3/4 + 2 1/4 .  I'm not sure why and then how to relay this reasoning to my students.  Any help would be appreciated!

 

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Re: Grade 5 Module 3 Lesson 14 Problem Set
by Admin User - Thursday, January 19, 2017, 5:44 PM
 

Great question! Here is how I think of two options for solving this problem:

Option A: Use the commutative property to move the numbers around so that "like" denominators are next door to each other. Then subtract each of the three other numbers one at a time until we get to the final answer of 1.

Option B: Start by using the commutative property to move the numbers around. Then use the associative property to subtract the first two numbers. Rather than subtracting each of the final numbers one at a time, we can add them together and then subtract all at once.

example

Using simpler numbers, 

10 - 5 - 3 could be solved by doing 10 - (5+3). This is because subtracting 5 and THEN subtracting 3 is the same as simply subtracting 8.

I hope this helps!