she gets the answer in her head and loses half the marks for no steps

first maths assessment of the term came back. every answer right, 11 out of 20. teacher has written 'show working' next to almost all of them. she says writing steps is slower and she already knows the answer. i get that it's how cbse marks, but how do you get a kid who does it in her head to slow down and write it out without her feeling punished for being quick. we did a practice sheet together last night and she wrote the answers first and then added steps backwards to fit them