It depends upon how deep you want it filled. For example, if you want it filled 6" deep, then the cubic feet would be 175 sq. ft X 0.5 ft. = 87.5 cubic feet. 87.5 cubic feet / 5 cubic feet/bag = 17.5 bags
An acre is a measure of area, not volume, thus it is not correct to say "1 acre deep". It makes no sense. An acre is 43559.66 square feet. So, the number of cubic feet in 1 acre of land that is 1 foot deep would be 43559.66 cubic feet.
I cu yd = 27 cu ft you cannot get square feet from cubic yards as square feet is area and yards volume - you need to know how deep the 400 sq feet is - in other words, volume. For example 400 sq feet that is 1 feet deep is 400 cubic feet; then divide by 27 to get cubic yards