The present study investigated test responses from 259 immigrant and non-immigrant school year 9 students in Sweden with the focus on how they solved two problems on fractions, one of them halving a fraction, in a test. The authors report three observations. Newly arrived second language immigrants seemed less likely to have the word 'half' in their Swedish mathematical vocabulary. Moreover, second language learners with longer experience of the new language connected the word 'half' with a division by two, but showed mathematical difficulties in correctly applying it to a fraction. A third finding was that the longer the experiences with Swedish school mathematics, the more likely both first and second language learners were to erroneously omit the percentage symbol, when choosing to use percentage representation of the fraction given in the test problem. The authors suggest seeing newly and early arrived second language immigrants as meeting different challenges. The newly arrived second language immigrants may know some mathematical concepts better and Swedish language less. In contrast the opposite seems to hold for second language learners with longer experience of the language of instruction.