Family C would only pay families A and B for doing the portion of work that was the responsibility of family C not for what their(A and B) individual responsibilities were. that is,for sample case total 9 hours of work had to be done (each family had to do 3 hours apiece).Now both A and B not only did their share of 3 hours but family A completed 2 hours of C's work and family B did 1 hour of C's work hence the ratio is 2:1. - Similarly for next sample B just did its assigned work while A did its assigned work and all the work assigned to C and hence get the full amount.