Even if you're admitted, you should ask yourself as to how much you can benefit from a quantitative program if your math background is weak. What you should have at a minimum is a year of calculus (upto and including partial derivatives, multiple integrals, and power series), at least one semester of linear algebra (upto and including eigenvalues and the spectral theorem), at least one course in ODEs (including simple numerical methods), one calculus-based probability course, one course in theoretical stastistics, and ideally some exposure to scientific computing (numerical analysis + coding in a high-level language). This is a bare minimum. Some exposure to PDEs would be an added benefit, as would some exposure to real analysis (would help with the theoretical arguments in probability, stochastic, and numerical analysis).

The one book I unequivocally recommend for calculus is Gilbert Strang's "Calculus." Make it your bible.