A randomized controlled trial investigated the research question: Do students who participate in online tutoring and a related mathematical game learn more about fractions than students who only have access to the game? Participants were 144 students from four schools, all serving low-income students with low prior mathematics achievement. In the Treatment condition, students received 20- to 25-minute tutoring sessions twice per week for an average of 18 sessions and also played the game. In the Control condition, students had access to the game, but did not play it often. Control students did not receive tutoring. Students were randomly assigned to a condition after being matched on pretest scores. The same diagnostic assessment was used as a pretest and as a posttest. The planned analysis looked for differences in gain scores (posttest minus pretest scores) between conditions. We conducted a t-test on the aggregate gain scores, comparing conditions; the results were statistically significant (t = 4.0545, df = 132.66, p-value < 0.001). To determine an effect size, we treated each site as a study in a meta-analysis. Using gain scores, the effect size was g = +0.66. A more sophisticated treatment of the pooled standard deviation resulted in a corrected effect size of g =0 .46 with a 95% confidence interval of [+0.23, +0.70].[Co-written with MenloEDU under a subcontract from Cignition. Additional funding from Schmidt Futures.]