Moving average is very simple method used to predict future based on the past results. It works well if there are no seasonal or cyclical elements included in the past data. It is calculated as an average of defined number ongoing periods.
Calculation of 3-month moving average for April: (255+260+279)/3 = 265
Calculation of 6-month moving average for July: (255+260+279+285+270+300)/6 = 275