There were 9 cards numbered 1 to 9. Four people A, B, C and D each collected two of them.
A said : “ The sum of my numbers is 6. ”
B said : “ The difference between my numbers is 5. ”
C said : “ The product of my numbers is 18. ”
D said : “ One of my numbers is twice the other. ”
What is the number on the remaining card ?
