Bottleneck process (BPP 78)

Corrie produces three products, X, Y and Z. The capacity of Corrie's plant is restricted by process alpha. Process alpha is expected to be operational for eight hours per day and can produce 1,200 units of X per hour, 1,500 units of Y per hour and 600 units of Z per hour. Selling prices and material costs for each product are as follows. Product Selling price; Material cost; Throughput contribution $ per unit $ per unit $ per unit X 150 70 80 Y 120 40 80 Z 300 100 200 Conversion costs are $720,000 per day What is the efficiency of the bottleneck process given the output achieved is 6,000 units of X, 4,500 units of Y and 1,200 units of Z? MY QUESTION: In the book’s answer, they convert hours to minutes and calculate that way, but I didn’t understand the logic behind it. Can you enlighten me, Sir?