the student needs to get comfortable with units and parts
Agreed. Knowledge in solving with units and parts is necessary because not all problems can be conveniently reduced to single variable like in this question.
Coz drawng models is too time consuming for exams
Unfortunately, for some students (eg. my daughter), this visual aid determines whether she can solve the problem or not. So at the beginning of P6, she was trained to draw model quickly, neatly and clearly without using model ruler. Also, even with units and parts question, it is still possible to draw model to help visualise the problem.
Take this question as example, I would draw a units and parts model (in free hand) like so…
Information in black ink are those transferred from the question, while those in red are deduced from the before model.
Of course, don’t use a red pen during exam! 😆