P(Brother or Sister) = .75 + .82 - .65 P(Brother or Sister) = .92 = 92% The reason you subtract the .65 is that scenario does not fit the rule. The rule is the probability of having a brother OR a sister. The .65 represents the kids who have both not either.
Any multiples of 4.5:1.5 would work because being proportional just depends on whether you can reduce to get the same answer of the first triangle. You could do anything like 9m and 3m or 27m and 9m
