We investigated the effects of proximal grouping of numbers, problem-solving goals to make 100, and prior knowledge on students' solution strategies in an online mathematics game. Logistic regression on 857 problem-level data points from 227 middle-school students showed that students were more likely to use productive solution strategies on addition and multiplication problems when proximity supported number grouping, 100 was the problem-solving goal, and students had high prior knowledge. Furthermore, when proximity and number goals did not support problem-solving, students with low prior knowledge were less likely to use productive solution strategies on multiplication problems than students with high prior knowledge. Findings demonstrate that the effects of perceptual and number features on solution strategies vary by students' prior knowledge.