Elements A and B react to form a compound Ax and By. Elements C and D react to form another compond Cp and Dq. What will be the formula of the compound formed, when A and D react?
The first compound is AxBy. So the valency of A is y.
The second compound is CpDq so the valency of D is p.
Now the compound made by A and D will be ApDy. The valency of A will written with D and the valency of D will written with A.(criss-cross rule)