As I was reading Thinking Fast and Slow by Princeton Professor and Nobel Prize recipient Daniel Kahneman, I came across the following mental exercise on p. 44
A bat and ball cost $1.10
The bat costs $1 more than the ball
How much does the ball cost?
If you answered 10 cents, you used your intuitive mind. You were too lazy or you rushed through the problem by trying to figure this out off the top your head. You did not “work to find the answer” You did not want to invest the time to write down a simple equation or check your work. If you answered 10 cents then the total cost would be $1.20 (.10 for the ball, $1.10 for the bat)
You should have made a simple equation x+x+1 = 1.10. Solve for x gets you .05. Checking your work the ball is .05 and the bat is 1.05 for a total value of $1.10.
Why is this important? Because the test makers will always fool the student who rushes. The student who rushes will always fall for the trick answer because he and she refuses to take a few extra seconds to think things through.