A catchment is idealized as a $25 \: km \times 25 \: km$ square. It has five rain gauges, one at each corner and one at the corner, as shown in the figure.
During a month, the precipitation at this gauges is measured as $G_1 = 300 \: mm, \: G_2=285 \: mm, \: G_3 = 272 \: mm, \: G_4=290 \: mm$ and $G_5=288 \: mm$. The average precipitation (in mm, up to one decimal place) over the catchment during this month by using the Thiessen polygon method is ___________