You used the wrong exchange rate when you converted RMB into pounds. Banks usually settle foreign exchange at the purchase price, so you should use the purchase price to calculate:

100 = 1473.05 yuan (purchase price), then 1 = 14.7305 yuan,

Then, 300 pounds = 300 x14.7305 = 4419.15 RMB-this is the cost price of RMB.

Because 100 USD =789.27 Yuan (purchase price), 1 USD =7.8927 Yuan,

Then, the revised dollar price is: 4419.15/7.8927 = 559.90 USD/metric ton.

The answer in your book is $599.89, which is also incorrect.