Mary had a whole cake and gave pretty 1/3 and moses 2/5...what is the remaining fraction of cake

To add the fractions $\frac{1}{3}$ and $\frac{2}{5}$, find a common denominator, which is 15.

$$\frac{1}{3} + \frac{2}{5} = \frac{1 \times 5}{3 \times 5} + \frac{2 \times 3}{5 \times 3} = \frac{5}{15} + \frac{6}{15} = \frac{5+6}{15} = \frac{11}{15}$$

$$1 - \frac{11}{15} = \frac{15}{15} - \frac{11}{15} = \frac{15-11}{15} = \frac{4}{15}$$

The remaining fraction of cake is $\frac{4}{15}$.