How many bits would need to be added to DES's key in order to quadruple its average brute-force cracking time?

1.    Using the English alphabet (i.e., mod 26 arithmetic) let plaintext = {p1, p2,... , pn} and corresponding ciphertext = {c1, c2,... , cn}. Suppose the encryption function is              ci = pi + 5 (mod 26). If you receive the ciphertext message CJPNZJAXVMYN, decrypt to recover the plaintext. What is the decryption function, and the recovered plaintext? What type of cipher is this? What are some weaknesses of this cipher?