The combined effects of diffusion and reaction in heterogeneous media govern the behavior of a wide variety of physical and biological phenomena, including the consumption of nutrients by cells and the study of magnetic relaxation in tissues. We have considered the so-called "trapping problem," in which diffusion takes place exterior to a collection of fixed traps while reaction occurs at their surface. A simulation technique for predicting the overall trapping rate for systems of partially absorbing spherical traps based on the first-passage spheres method is presented. Using data obtained by applying this simulation technique, we then consider the problem of mixtures of partially absorbing traps. By hypothesizing a method for reducing a general mixture of traps to a mixture of perfect absorbers and perfect reflectors (i.e., reducing the dimensionality of the space of variables), we are able to accurately predict the effective surface rate constant and the trapping rate for an arbitrary mixture of partially absorbing traps. Remarkably, we find that a single, nearly universal curve allows accurate predictions to be made over a wide range of trap volume fractions and even for different trap distributions.