12．口算．
0 138891 138899 138905 138909 138915 138917 138921 138927 138929 138935 138941 138945 138947 138951 138957 138959 138965 138969 138971 138975 138977 138981 138983 138985 138986 138987 138989 138990 138991 138993 138995 138999 139001 139005 139007 139011 139017 139019 139025 139029 139031 139035 139041 139047 139049 139055 139059 139061 139067 139071 139077 139085 199269
| 15÷0.25= | 1.5×$\frac{2}{5}$= | 24×$\frac{3}{8}$= | 2.4+1.6= |
| $\frac{5}{12}$÷$\frac{5}{6}$= | $\frac{5}{7}$×$\frac{21}{25}$= | $\frac{5}{9}$×1.8= | 2.5×0.4= |