There are ten cards numbered from 0 to 9. Five of these are face down in a row on the table.
The numbers on the first two cards add to the first key number.
The numbers on the second and third cards add to 9,
the numbers on the third and fourth cards add to 11,
and the numbers on the fourth and fifth cards add to 16.
What number is on the last card? This number is the second key.