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b.There were 158 paper dolls at Barbara’s house. Her sister brought some more. Then there were 201. How many paper dolls did her sister bring?  (359 paper dolls)

the student added all the dolls together. They might have struggled to read the word problem to see that they were asking what the difference between the starting point was to the new point.

You could also add on: You start with 158 then add how many to get to 201?

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a.Alex has 53 nickels and some dimes. He has 71 coins all together.  How many dimes does he have?  (22 dimes)

b.There were 158 paper dolls at Barbara’s house. Her sister brought some more. Then there were 201. How many paper dolls did her sister bring?  (359 paper dolls)

c. Carl gives 38 carrot sticks to his friend Monica.  Monica has 75 carrot sticks on her party plate and puts the 38 carrot sticks in her ice box for later.  How many does carrot sticks does she have?  (103 carrot sticks.)

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I did it two different ways, a normal subtraction and then a part-part-whole problem. I think the student got 22, because the rounded up the 71 to 75 and didn't re adjust the problem.

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So when I did the math, I got 43. I believe that the student added the two numbers together and got their answer when they needed to subtract instead.
To help a student understand their mistake, I would ask them to model their problem using base ten blocks. I would have them highlight important parts of the problem and reread the question. Then I would have the students show me using the blocks what the problem looks like and how they might solve this. I showed an image of what they might get above.

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I started with adding 10, to get an easier number, and then I add 7 to get an even number. Then I just added one. Finally, I just took the last number to get to the answer.

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I think the problem is place value. When Carl added 38 and 75, he forgot to add the 1 that was carried over from the ones place to the tens place

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75 + 38 = 113 carrot sticks
This student must have had an error when doing the standard algorithm for adding and had difficulty with the place value. The invented strategy that I would recommend is "Add Tens, Add Ones, and Combine" (shown in picture).