4. [4 points] Given the list of primes [2, 5, 11, 13), find the residues of the following numbers modulo each prime in the list. (a) 981 (b) 456 5. [8 points Let n = 7. (a) Find a number r such that 3x = 1 mod 7 (b) What is the inverse of 3 modulo 7? (c) Is there a number x such that 2x = 1 mod 4? Explain. (d) Is 2 invertible modulo 4