You have the following sequence:
115, 103.5, 93.15, 83.835,
In orde to determine if the previous sequence is arithmetic or geometric, you consider that in an arithmetic sequence each term is obtained by adding to the previous term a definite constant. In a geometric sequence each term is obtained by multiplying the previous term by a constant factor.
In this case, you can notice that each term is the result of the product between the previous term and 0.9. Or which is the same the division between two consecutive terms is equal to 0.9. In fact you have:
115(0.9) = 103.5
103.5(0.9) = 93.15
93.15(0.9) = 83.835
and the fifth term is
83.835(0.9) = 75.4515
Hence, the given sequence is a geometric sequence and its fifth term is 75.4515