Our math teacher said that cross multiplication method is not a logical method to solve in!!! even if the answer is right!!!
so is he right?
If you are referring to the property in algebra that implies provided of course b,d are nonzero... then yes, this is a valid and completely sound property of real numbers.
Perhaps what your professor means by "illogical" is that cross-multiplication is not a fundamental property or operation in algebra. The correct and logical step is to multiply both sides of the equation by . The term "cross multiply" is not so much a mathematical tool as it is a teaching tool, to help young (particularly math-hating) students remember how to proceed when faced with this type of problem.