My source tells me that people come up with quite a wide range of answers:

Unfortunately, it does not explain what kind of errors they make.

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120

 

8 minutes ago, mistermack said:

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120

 

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No, it is not a correct answer. I wonder, how did you get it, if you don't mind.

 

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Oh yea, using my method, it should have been 60

 

9 minutes ago, mistermack said:

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Oh yea, using my method, it should have been 60

 

This is correct. +1

The diagram is misleading as a 20° angle is not greater than a 40° angle... just saying.

2 hours ago, Growl said:

The diagram is misleading as a 20° angle is not greater than a 40° angle... just saying.

It’s not supposed to be a visually faithful depiction, so that the picture doesn’t reveal the answer. You’re supposed to use math skills rather than measurement.