Consider the following pattern of numbers:

8	10	15	13
6	5	7	4
4	6	8	8
6	11	16	?

What is the number at ? in the above pattern ?

The correct answer is 17.

To find the pattern, let us examine each column carefully. The table is arranged as follows:

Row 1: 8   10   15   13
Row 2: 6     5     7     4
Row 3: 4     6     8     8
Row 4: 6   11   16   ?

The governing rule for each column is:

$\text{Row 4}=\text{Row 1}-\text{Row 2}+\text{Row 3}$

Let us verify this rule across the first three columns:

Column 1:
$8-6+4=6$   ✓ (matches Row 4, Column 1)

Column 2:
$10-5+6=11$   ✓ (matches Row 4, Column 2)

Column 3:
$15-7+8=16$   ✓ (matches Row 4, Column 3)

The rule is confirmed consistently across all three columns. Now we apply it to Column 4 to find the missing value:

Column 4:
$13-4+8=17$

Therefore, the missing number ? is 17.