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Part A

 33.2 degrees F

Explanation: Adding on a negative is the same as subtracting. So 72.3 + (-39.1) = 72.3 - 39.1 = 33.2

Part B

70 +  2   +  0.3    + (-30) + (-9) + (   -0.1   )

Explanation:

Think of 72 as 70+2. Furthermore, think of 72.3 as 70+2+0.3; we just break the number up into its corresponding digits (adding zeros when needed). The 7 is in the tens place, the 2 is in the units or ones place, and the 3 is in the tenths place.

Similarly, we have 39.1 break down into 30+9+0.1, in which all three terms are made negative to represent -39.1

Part C

  70 + (-30) + 2 + (-9) + 0.3 + ( -0.1  )  

Explanation: Arrange the tens place value items to be next to each other. Same goes for the units place value, and also the tenths place value.

Part D

 [70 + (-30)] +  [ 2 + (-9) ] + [ 0.3 + ( -0.1  ) ]

Explanation: Take the result of part C and surround each pair of terms in square brackets to show how the terms pair up.

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