It's been a couple of decades now, but that's still something I remember from attending government schools. More than once my math tests or homework were marked down because, "you didn't show your steps". Just sheer idiocy! The fact that someone can work something out in their head and get the right answer is the surest indication that they have mastered the subject matter.
An answer without presenting the logic just as likely a signal that the student cheated to get the answer. I won't give full credit for an answer in math or chemistry unless the student can demonstrate, either on the test or face-to-face, that they knew what they were doing. Most of the time they can't. It's not idiocy to demand a demonstration of reasoning in the face of the student culture of today.
As well you should have been. Similarly, you are expected to write in complete sentences, capitalizing proper nouns and the first words of sentences.
Mathematics courses are about methods, not answers. Sans showing steps, there is no way to tell if the student even has an idea about how to solve a problem. The student may have got the answer by working it out, hearing it on the radio, seeing the answer on a nearby student's paper, or guessing. Also, (at least in some of my courses), partial credit would be available for correct methods with some penalty for arithmetic mistakes.