There are N numbers available: from 1 to N.

The rows of a 8 by C array are each filled with C numbers,

each row ending up with C different numbers.

A number may appear a maximum of 4 times,

in other words, in a maximum of 4 rows.

Every 2-row-combo has a minimum of 2 numbers in common.

What is minimum N?

Solution is minimum N = 12; this is the resulting array (C = 6):

1: 01 02 05 06 07 08

2: 01 02 05 06 07 08

3: 01 02 09 10 11 12

4: 01 02 09 10 11 12

5: 03 04 05 06 09 10

6: 03 04 05 06 09 10

7: 03 04 07 08 11 12

8: 03 04 07 08 11 12

OK. Changing the conditions a bit (or making it tougher!):

There are N numbers available: from 1 to N.

The rows of a 20 by C array are each filled with C numbers,

each row ending up with C different numbers.

A number may appear a maximum of 10 times,

in other words, in a maximum of 10 rows.

Every 5-row-combo has a minimum of 2 numbers in common.

What is minimum N?