So you have 115 as a last term and 55 as a second term to find the missing terms
Use this formula.
Last term= second term+Number of the diffidence
115=55+4D
Replace the 55 to the other side and it will be -55
115-55=4D
115-55=60
60=4D
Devide both sides by 4
D=15
55+15=70
70+15=85
85+15=100
115
The missing terms are 70, 85, 100