Four digit numbers are formed by using the digits 1, 2, 3, 5 without repetition of digits. How many of them are divisible by 4?

A number is divisible by 4 if its last two digits form a number divisible by 4. From {1,2,3,5}, possible last-two-digit combinations divisible by 4: 12, 32, 52. That's 3 combinations. For each, the first two digits can be arranged from the remaining 2 digits in 2! = 2 ways. Total = 3 × 2 = 6.