Throughput return per hour

Hi John, this question is from BPP Mock exam - question 4. I got the right answer correctly by calculating the marginal cost and then dividing it by bottleneck hour which is 0.1 (6 minutes converted into hour). but later realized that the 6 mins was actually divided by the conversion costs of $60. why did we divided it by the conversion cost? I'd thought that in order to get the return per hour we will need to convert the 6 mins to hour. Please can you help