If the roots of the equation 4x² - (5k + 1)x + 5k = 0 differ by unity, then which one of the following is a possible value of k?

Consider the digits 3, 5, 7, 9. What is the number of 5-digit numbers formed by these digits in which each of these four digits appears?

How many distinct matrices exist with all four entries taken from (1, 2)?

If i = √-1, then how many values does i^-2n have for different n ∈ ℤ?