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We developed and validated intervention aimed at enhancing numeracy (the ability to understand and use the concept of probability and statistical information) and decision making. One hundred and twenty-two participants were randomly assigned to a Mental Number Line Training condition (MNLT) or an Arithmetic Training Active Control condition (ATAC). Response mode (a slider anchored within the current response range vs. a numerical keyboard) was the only, and essential, difference between experimental conditions. After MNLT, participants developed a more precise mapping of numbers onto the mental number line. However, the results regarding the transfer of this effect to performance in other cognitive tasks were mixed. We observed the transfer to an ability to quickly estimate the sum of numerical quantities (measured by the precision of estimates for the total price of everyday products), but we did not find evidence that MNLT, as compared to the control condition, improved performance on numerical competencies measures or decision making tasks. Interestingly, we found that both MNLT and ATAC resulted in higher subjective numeracy as well as normatively better financial decisions, and valuation of risky prospects. Developing more exact symbolic-number mapping may be a promising direction of future research on improving decision making skills.