Let the mean and variance of the frequency distribution
\matrix{
{x:} & {{x_1} = 2} & {{x_2} = 6} & {{x_3} = 8} & {{x_4} = 9} \cr
{f:} & 4 & 4 & \alpha & \beta \cr
}
be 6 and 6.8 respectively. If x₃ is changed from 8 to 7, then the mean for the new data will be :