How many different 4-letter radio station call letters can be made if (a) the first letter must be G, X, P, or L and no letter may be repeated? (b) repeats are allowed, but the first letter is G, X, P, or L? (c) the first letter is G, X, P, or L, there are no repeats, and the last letter is R? (a) If the first letter must be G, X, P, or L, and all the letters in the radio station call letters must be different, then there are 4-letter radio station call letters. (b) If the first letter must be G, X, P, or L, and letters can be repeated, then there are radio station call letters. (c) If the first letter must be G, X, P, or L, and the last letter must be R and all the letters in the radio station call letters must be different, then there are 4-letter words.