Final answer:
The draft of the river barge increases by 4 ft when loaded with coal.
Step-by-step explanation:
The subject of this question is Physics. The question asks about the draft (depth of submergence) of a river barge. The barge is 28 ft. wide and 90 ft. long. When unloaded, its draft is 5 ft. and with the load of coal, the draft increases to 9 ft.
To find the change in draft caused by the load of coal, we can subtract the unloaded draft from the loaded draft: 9 ft - 5 ft = 4 ft.
Therefore, the load of coal causes the draft of the barge to increase by 4 ft.