In this study, we investigate the effect of problem text on solving th
ree types of difference problems by first graders. COMPARE problems we
re significantly harder than EQUALIZE and WON'T GET problems. In terms
of solution strategies, EQUALIZE problems were generally solved by an
ADD-ON strategy; and the WON'T GET problems, by a MATCH strategy, whi
ch reflected the coordination of two mental number lines as the proble
m representation. No one strategy was used more often for the COMPARE
problems, indicating that the COMPARE wording did not facilitate such
coordination. These results suggest the coordination of mental number
lines as the crucial component in solving difference problems.