Let say that the answer is 40 degree as degrees N of E and 50 degree as E of N and that you assigned unit 1 as 1 for degrees N of E and 2 as degrees E of N.

You change your formula so that it calculates the second option as half the response i.e. 25 for unit degrees E of N so that with the correct unit, this will give the 50 degrees.

So the student could give 4 possible answers

40 degrees N of E (value 1) that is 100% grade formula = rad2deg(asin(sin(2.356)*({z}/{y})))

50 degrees E of N (value 2) that is 100% grade formula = 0.5*(90-(rad2deg(asin(sin(2.356)*({z}/{y}))))

For the bad unit you could create the two other answers with for example a 80% grade

40 degrees E of N (value 2) that is 80% grade formula = 0.5* rad2deg(asin(sin(2.356)*({z}/{y})))

50 degrees N of E (value 1) that is 80% grade formula = (90-(rad2deg(asin(sin(2.356)*({z}/{y}))))

If I have set everything Ok this should do what you need.

Pierre

P.S. I am confident that you will not set the values so the the response is 45 degrees .