Not all problems on multiple choice tests are designed to be solved. Some are there to test concepts that, if you had the knowledge, you could eliminate all the other answers.
Let's try an example using calculating a tip.
Find the dollar amount of a 10% tip on a $28 bill.
a. $5.40
b. $5.60
c. $2.80
d. $0.28
The knowledgeable student knows he only needs to move the decimal place one place to the left. So the actual numerals are not changing. This means that a and b are obviously incorrect. Now consider c and b. The numerals are the same as the $28 amount. But in c the decima point was moved 1 place to the left while in d it was moved 2 places to the left.What is the rule when multiplying by factors of 10? Only move the decimal one place to the left.
So, the teacher is likely only giving you the time to really think about two answers since the first two are so obviously wrong.
How do teachers choose the other wrong options on test? I chose b because it represented 20% If we had been working with lots of 20% tips in class a lazy reader or rigid thinker might automatically go for choice b. I just learned something about that student. I chose a because it represents a math error on the 20% tip. This means all of the above, plus their calculation skills are poor.
How about this problem?
Which represents 80% of 200?
A 16000
B 280
C 1000
D 160
You should be able to look at that and immediately know the answer is D because it's the only answer that is less than the original number 200. When you multiply a whole number by a fraction, the answer is always smaller. As a teacher, I dont want you to solve that problem, I just want to know that you understand that concept.
Maybe you can figure out the errors in the wrong options and then the concepts that I would then have to reteach.