What is the sum of all four-digit numbers formed by using all digits 0, 1, 4, 5 without repetition of digits?

If $x, y$ and $z$ are the cube roots of unity, then what is the value of $xy + yz + zx$?

A man has 7 relatives (4 women and 3 men). His wife also has 7 relatives (3 women and 4 men). In how many ways can they invite 3 women and 3 men so that 3 of them are man's relatives and 3 of them are his wife's relatives?