Is is possible to study mathematics PHD with another bachelor degree like Business Administration?
The short answer is: "yes, but it won't automatically be easy"

First, every department of every school in every major has a set of preferences - things they like to see in an applicant. This list includes GPA, GRE, sometimes subject-specific GRE, specific coursework, specific reference letters, and the rarely-ever-listed undergraduate research/term papers that lots of people here give great advice about but that I didn't previously know was a 'thing'.

Looking over the admittance pages of prospective programs can give you an idea of this at times. In your case, I looked at a few schools math departments I'm familiar with, and using this one as an example because its not terribly atypical (but you should research your own preferred institutions): University of Wisconsin Mathematics


  1) A GPA of at least 3.0 overall and substantially higher in mathematics courses.
  
  2) 18 or more credits of mathematics beyond calculus. The 18 credits should include at least one semester of both rigorous analysis
  (sometimes called advanced calculus) and abstract algebra, and
  preferably two semesters. Advanced undergraduate courses in other
  topics further strengthen your application.
  
  6) A “Supplemental Application Form” is required after submitting your
  online Graduate School application. It is important that you include
  all advanced mathematics courses you have completed, are currently
  enrolled in, or intend to take...
  
  7) Three letters that address the question of mathematical promise.
  These letters should be from mathematics faculty, or other faculty
  familiar with the demands of graduate work in mathematics...


Now keep in mind that any published outlines are a "wishlist", and they don't automatically toss anyone. But every area you don't fit requires explanation or some other counter-balancing factor. Some programs give a list of x requirements, then say they'll waive 1-2 of them without further explanation needed, etc.

If you are missing all of these, then you need to look at a different program, but you might find no program that's a fit. If this happens, that doesn't mean you can't do something, just that you might have to go about it sideways or backtrack a bit.

If you are missing just a few things, like a course or two, some programs have probation where they'll tentatively admit you on the demand you complete X courses on your first semester and get at least Y GPA, etc. 

Alternatively, you might need to make arrangements to take advanced math courses at an undergrad/graduate level. It's harder to get financial aid for this sort of thing (you can't qualify for a Pell grant anymore, etc), but you can still get loans or pay for it outright. If you need this route, you need to talk personally with professors in advance to discuss what your goals are and make any possible arrangement to do extra work, write an academic research paper, etc. You need to find people you can have enough contact with to be willing to go to bat for you in writing strong reference letters if you don't already have math professors who are familiar with your past work. Join math clubs, go to math study labs, volunteer, whatever you can manage.

Before you go running off and doing any of this, however, you should make contact with math faculty at your present school and/or at a target school, and get an opportunity to talk with them about what you want to do and get their advice. Sometimes just showing "mathematical promise" can get a magic wand waived over your application, which maybe you already have - but only someone familiar with what you've done can tell you that. If you have a piece of mathematical "best work" from your most advanced class, bring it with you to such a meeting.

Once you've gotten a few honest appraisals of where you stand, be kind to yourself in looking for programs that will fit YOU, not some imaginary ideal program for someone else.

Finally, one last piece of advice. Keep your mind open to hybrid possibilities. There are whole fields of study you might really like that you aren't aware of, including applied mathematics study offered by business schools, economic fields that emphasize mathematical theory and/or application, etc.

Sometimes we have to try to mold ourselves to fit an existing mold a little better, but sometimes its trying to fit a square peg into a round hole. Talk to people and investigate the possibilities, get a clearer understanding of your relative shape, and seek out openings with geometry congruent to yours. Good luck in your journey!