Sandy finished 3/8 of her homework before supper and 4/8 after supper. What fraction of her homework did she finish in total?

$$\frac{3+4}{8} = \frac{7}{8}$$

Sandy finished $\boxed{\frac{7}{8}}$ of her homework in total.

b) What fraction of her homework will Sandy still have to do before school the next day?

Subtract the fraction finished from the total.

$$1 - \frac{7}{8}$$

$$\frac{8}{8} - \frac{7}{8} = \frac{8-7}{8} = \frac{1}{8}$$

Sandy will still have $\boxed{\frac{1}{8}}$ of her homework to do.

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Hlengiwe has $2\frac{5}{8}$ bags of sweets and Craig has $3\frac{7}{8}$ bags of sweets. If they combine their sweets, how many bags of sweets will they have together?

Add the whole numbers and the fractions separately.

$$2\frac{5}{8} + 3\frac{7}{8} = (2+3) + \left(\frac{5}{8} + \frac{7}{8}\right)$$

$$5 + 1\frac{4}{8} = 6\frac{4}{8} = 6\frac{1}{2}$$

They will have $\boxed{6\frac{1}{2}}$ bags of sweets together.

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Lucas spent $\frac{5}{12}$ of his salary on rent and another $\frac{6}{12}$ of his salary on food.

a) What fraction of his salary did he use for rent and food combined?

$$\frac{5+6}{12} = \frac{11}{12}$$

Lucas used $\boxed{\frac{11}{12}}$ of his salary for rent and food combined.

b) What fraction of his salary was left over after these expenses?

Subtract the fraction spent from the total.

$$1 - \frac{11}{12}$$

$$\frac{12}{12} - \frac{11}{12} = \frac{12-11}{12} = \frac{1}{12}$$

Lucas had $\boxed{\frac{1}{12}}$ of his salary left over.