In this case, the ore yield 64% iron and you need 896 lb of iron. From this description you can get an equation like this:
Iron= iron ore mass * 64%
Then, to get 896 of iron the calculation should be:
Iron= iron ore mass * 64%
896 lb= iron ore mass * 0.64
iron ore mass= 896 lb/ 0.64= 1400lb
