11. Matchsticks can be used to write digits, as shown in the diagram. How many different positive integers can be written using exactly six matchsticks in this way?
A 2
B 4
C 6
D 8
E 9
Solution
C

The number of matchsticks used for each of the digits 0 to 9 is $6,2,5,5,4,5,6,3,7$ and 6 respectively. Hence, the positive integers that can be made using exactly six matches are 6,9 , $14,41,77$ and 111. Therefore 6 different positive integers can be made.