@Synonyms it is 1/8 but i dont know why exactly
@CrazinessISlife assuming I’m interpreting the question correctly, one way to do it is to look at the cross-sectional areas.
Or you can assume that the side length of the cube is, say, 2, and compute the volumes of the two solids formed.
@CrazinessIslife Like MITer94 said, the easier solution is to plug in 2 the side length of the cube and do the math. To elaborate:
Let the midpoints of B1B, A1A, BC, AD be called M, N, P, Q respectively.
=> MB = BQ = NA = AP =1.
Then, tilt your head sideways and you’ll see that we’ve got a prism with 2 triangles as bases on our hand: MBQ.NAP.
AB is perpendicular to plane (B1BCC1) (ABCD.A1B1C1D1 is a cube).
=> MNQ.NAP is a right prism.
=> height (h) = length of sideways edge = MN.
We also have: MB=NA=1; MB//NA (ABCD.A1B1C1D1 is a cube)
=> MNAB is a parallelogram => MN = AB = 2 => h = 2
A (MBQ) = (1/2) * MB*BQ = 1/2
V (prism) = A (MBQ) * h = 1/2 * 2 =1.
V (cube) = 2^3 =8.
V (prism) /V (cube) = 1/8.
=> V (left over) / V (cube) = 1 - 1/8 = 7/8.
@Synonyms i dont understand it well 
…am confused bec idk how u r labeling the cube idk where is A1 B1 etc so i cant draw it…they should be three parallelogram and two triangles right?
when i do it i add the three parallelograms with the two traingles…
It’s a bit tricky to describe the 3D shape using words, but here is what @Synonyms meant:
Draw a picture of a cube.
Label one face (e.g. the bottom face) ABCD, in order.
Label the opposite face A1B1C1D1 in the same order so that AA1 is an edge of the cube, BB1 is an edge of the cube, etc.
The above description should be unambiguous.
The plane mentioned is determined by the midpoints of AD, CD, A1D1, and C1D1.
If you draw it correctly, the cube should be split into a smaller triangular prism (which is 1/8 the volume of the cube) and a larger pentagonal prism (7/8 the volume of the cube).
Describing exactly what it looks like on this forum without a picture is difficult, but the problem is fairly easy if you draw it out.
i did like that but i dont get why its 1/8 i only get that its 1/8 when i divide the cube into equal parts…but with puttin numbers why calculated 1 triangle and 1 parallelogram when they are three parallelogram and 2 triangles.???..theres a pic for the problem but idk the name of the test to get it in a link
@CrazinessISlife I honestly can’t understand what you’re saying. Use correct grammar and unambiguous language. There are no “parallelograms” involved (except for rectangular faces).
Assume the side length of the cube is 2. The cut splits the cube into a triangular prism (with height 2, and a right isosceles triangle base of leg length 1), and a pentagonal prism.
The volume of the cube is 2^3 = 8. The volume of the triangular prism is (base area)(height) = (11*(1/2)) * 2 = 1. The volume of the triangular prism is 1/8 that of the cube.
@MITer94 sorry i am not a native speaker
i got what u r trying to say thanks and i appreciated it
and i meant i got it in another way without using numbers i divide the cube into equal parts ( equal triangles that are the same size as in the triangular prism) and so i got it directly 1/8…is this a correct way of doing it?
It won’t work for similar problems, but it seems like a valid solution for this problem specifically. That is one way to solve the problem. 
^^ One comment regarding not being a native speaker.
You are now busy with preparing for the SAT and work through math problems. That is only one part of the equation. You should take advantage of every opportunity to sharpen your skills, including writing. This is not a matter of vocabulary but a matter of using proper grammar and punctuation. Your objective is to attend a college in the US, and you start working on that part without delays.
Hence, make the effort to edit your posts. Use punctuation and capital letters to mark your sentences. And, above all, please abandon the cryptic dewdspeak such “u r” and related shortcuts.
^^agreed 100%. As a math major, any time we see really vague or incorrect descriptions, it becomes quite difficult to read. For example, in your solution, it is better to say that you cut the cube into equal triangular prisms, rather than equal triangles.
This is not just for math problems either - when you are writing papers, for example, you want to avoid bad grammar and ambiguous or inconsistent notation.
To be clear, the expectation is NOT to have perfect posts. None of us write perfect posts as we might rely on smart phones or even dictate a reply on a bus. I tend to forget words as I think faster than I type! 
It is, however, important to focus on “getting better” by practicing. Students should not hesitate to write imperfect posts if that is the best they can do. Over time, others will help by helping understanding differences (prism vs triangle as MIT did) and ALL of that is a helping tool. It is not meant to be condescending in any way or form.
@MITer94 @xiggi thanks alot will try = )
No problem.
PS: It’s “a lot.” Alot is never correct in written English, although allot is an English word (e.g. allot someone time).
Ha! I knew that someone would be all over “a lot”. Took longer than I thought it would