when I divide two fractions example

[1/2] / [3/5] I know that I have to invert and then multiply

[1/2]*[5/3] so my answer will be 5/6

My question is why do I invert then multiply inorder to get the correct answer, Why cannot I just say that it is 3/10, why do we need to invert and multiply, why is this the rule that we must invert then multiply?