Question in Unit 1

Dear John, How are you? I hope you are doing well and safe. Q-The fastest time in which a batch of 20 "spicy meat special" sandwiches has been made was 32 minutes, with no hold-ups. However, work studies have shown that, on average, about 8% of the sandwiches makers time is non-productive. Required, what is the attainable standard time of one sandwich? *The answer as per Kaplan book Ideal time 32 minutes plus non-productive time 2.8 non - productive idle time so total attainable time required = 34.8 minute. *My answer Ideal time 32 minutes Plus non-productive time (32 ideal time * 8%)=2.56 so the total attainable time required =32 minutes + 2.56 minutes = 34.56 Minutes. 1-Could you tell me why my non-productive time calculation is wrong? 2- when should I do use his way of work-out and when should I do use my way of work-out of calculating non-productive time because I am very confused? Thank you in advance.