The combined mean for two sets will depend on also the means for the each respective set:
"x_{a+b} = \\frac{n_ax_a+n_bx_b}{n_a+n_b}" ,
where:
"x_a" = the mean of the first set,
"n_a" = the number of items in the first set,
"x_b" = the mean of the second set,
"n_b" = the number of items in the second set,
"x_{a+b}" = the combined mean.
Using the information we have,
"n_a = 3; n_b = 6, \\\\\nx_{a+b} = \\frac{1}{9}(3x_a+6x_b) = \\frac{1}{3}(x_a+2x_b)."
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