Abstract: This study examined the correlation between problem-solving ability, mantal- calculation ability and estimation ability of primary 5 pupils. It also attempted to determine the strategies used by high ability pupils in the estimation test. 567 pupils from 4 Singapore school were administered a problem-solving, a mental- calculation and an estimation test. Results from quantitative analyses showed that the scores of problem-solving mental-calculation and estimation were significantly correlated. Males performed significantly better than females in estimation test. Interview with the pupils indicated that the good estimator were able to develop their own strategies and deviate from rules leaned in the classroom.