25 percent can be converted to 1/4 and 4 x 1/4 = 1 and/or whole
Now since 45 marbles = 25 percent/1 /4 of WHAT is in the jar now we can multiple 45 x 4 (times 4 to make the 1/4 into a whole) which gives up 180 marbles total.
Hopefully I explained it in a way you can understand
To maximize area and minimize perimeter we make the dimensions as close to equal as possible. Since we only need fencing on 3 sides, 120/3 = 40; we can use 40 for each side.
