def ComputeRho(A, nr, A_tot):
    """
    Compute the ratio of area of a reinforcement to area of a section.

    @param A (float): Area of reinforcement.
    @param nr (float): Number of reinforcement (allow float for computing ratio with different area;
        just convert the other areas to one and compute the equivalent n).
    @param A_tot (float): Area of the concrete.

    @returns float: Ratio.
    """
    return nr * A / A_tot