function combineMeans(mean1, n1, mean2, n2) {
    return (mean1 * n1 + mean2 * n2) / (n1 + n2);
}
When combining two lists of values for which one already knows the means,
one does not have to necessary recompute the mean of the combined lists in
linear time. They can instead use this function to compute the combined
mean by providing the mean & number of values of the first list and the mean
& number of values of the second list.

@since 3.0.0
@param {number} mean1 mean of the first list
@param {number} n1 number of items in the first list
@param {number} mean2 mean of the second list
@param {number} n2 number of items in the second list
@returns {number} the combined mean

@example
combineMeans(5, 3, 4, 3); // => 4.5