Given our defaults that 2 can live as cheaply as 1.6 and that it costs 70% of an adult standard of living to give a child an equivalent living standard, then the economy of joint living, x, is such that 2^x = 1.6, i.e., x = log 1.6 / log 2 = 0.6781.
The general formula is the (number of adults + number of children * 0.7)^0.6781
So, the adult equivalent of two adults and one child is (2 + 0.7)^0.6781 = 1.961,
of two adults and two children is (2 + 1.4)^0.6781 = 2.293, and
of two adults and three children is (2 + 2.1)^0.6781 = 2.603.
The adult equivalent of one adult and one child is (1 + 0.7)^0.6781 = 1.433,
of one adult and two children is (1 + 1.4)^0.6781 = 1.811, and
of one adult and three children is (1 + 2.1)^0.6781 = 2.154.