Does anyone knows the answer and how to solve this question about Shannon's entropy?

A file contains only characters belonging to the set {A, B, C, D}. The relative frequencies of characters A, B C and D are 4/10 , 3/10 , 2/10 and 1/10, respectively. The file data will be compressed before transmission. According to Shannon's entropy, the average number of bits to encode each symbol is:

a) 2 b) 4 c) 1,15 d) 1,50 e) 1,85