Area can't be in "feet"; I'll assume it is "square feet".
Solve two simultaneous equations:
Perimeter: 2(a+b) = 100
Area: ab = 400
Solve perimeter equation for one variable:
2(a+b) = 100
a+b = 50
a = 50 - b
Substitute this in the other equation:
ab = 400
(50-b)b = 400
50b - b2 = 400
b2 -50b+400 = 0
To continue, use the quadratic formula, factoring, or complete the square. In this case, factoring is easy, so:
(b-40)(b-10) = 0
"b" can be either 40 or 10. Replacing in the equation ab = 400, "a" is either 10 or 40.
400feet
This question cannot be answered in a sensible way. A square yard is a measure of area, with dimensions [L2]. The measure of the hole / trench is in cubic feet which is a measure of volume, with dimensions [L3]. Basic dimensional analysis teaches that you cannot convert between measures with different dimensions without additional information.
1000 square feet to paint? You can paint 400 feet squared with one gallon. so you need to know how high the wall is 8 feet high by 1000 feet 8x1000 = 8000 square feet divided by 400feet per a gallon = 20 gallons one coat. just x the amount of coats you want to do and your good to go. You realy need to measure each wall and go HxL divided by 400feet per a gallon and this will tell you how many gallons will be needed to paint each wall and corner.
This will depend greatly on the filter you want to use. A flat filter will need to be larger than a pleated filter. Determine what efficiency you need from your air filter. Look up the maximum "face velocity" across the filter, most are around 500fpm. Divide the blower cfm by the filter fpm and you will end up with ft2 or filter needed. Let's do an example of a 2000cfm blower with a filter that requires a face velocity of 400feet/min. 2000ft3/m / 400ft/m = 5ft2 or (1)24"x30" filter.
Mammoth Cave near Cave City, Kentucky, is the world's longest cave. More than 390 miles of passages have been mapped. Mammoth Cave is three times longer than its nearest competitor. Not only is the cave the longest in the world, but cave mappers believe the cave system will eventually prove to be 560 miles (900 km) long. The second longest cave in the world, Optimisticeskay Cave, in the Ukrainskaya region of the C.I.S. (former Soviet Union), is only 143 miles (230 km) long.It is 4.6 miles long.