A catchment area has seven raingauge stations. In a year the annual rainfall recorded by the gauges are as follows: Station Rainfall (cm) P Q R S T 130.0 142.1 118.2 108.5 165.2 U 102.1 V 146.9 For a 5% error in the estimation of the mean rainfall, calculate the minimum number of additional stations required to be established in the catchment.