Look at a digital watch  a decimal digit is displayed by turning on and off seven segments.
Below are three lines: the first is a list
of the ten digits, the second is a list of the number of segments turned off to display the decimal digit above, and the third is a list
of the number of segments turned on to display the decimal digit.
Clearly, the number in the second line
plus the number in the third
line is seven.
0 1 2 3 4 5 6 7 8 9
1 5 2 2 3 2 1 4 0 1
6 2 5 5 4 5 6 3 7 6
a) Two is the only digit that equals the number of segments turned off to display itself.
b) Four, five and six all equal the number of segments turned on to display themselves.
How could I have worded the problem such that the difficulty level would not be d0?
