Daniel Kahneman - Thinking Fast and Slow
1) A bat and ball cost $1.10.
2) The bat costs one dollar more than the ball.
3) How much does the ball cost?
x+y=1.1; x=cost of the bat, y=cost of the ball.
y=x+1; substituting for y
x=0.05; the cost of the ball
y=1.05; the cost of the bat
The right answer is 5 cents.
The intuitive, appealing, and wrong number is, of course 10 cents.