How many two-digit positive integers can be formed from the digits 1, 5, 6, and 8 if no digit is repeated?

Total Number of digits = 4

We have to fill two places using the 4 digits without repeating the digits.

The first place digit can be filled in 4 ways. When first place is filled, one number is already taken, so now we can fill the second place using remaining 3 digits (3 ways)

So, the total number of ways of forming 2 digit numbers = 4 x 3 = 12 ways