Tính.

a) \(\frac{{ - 9}}{{15}} + \frac{{ - 6}}{{15}} = \frac{{ - 9 + \left( { - 6} \right)}}{{15}} = \frac{{ - 15}}{{15}} =  - 1\)

b) \(\frac{{15}}{{20}} + \frac{7}{4} = \frac{{15}}{{20}} + \frac{{35}}{{20}} = \frac{{15 + 35}}{{20}} = \frac{{50}}{{20}} = \frac{{25}}{2}\)

c) \(\frac{{ - 4}}{{16}} + \frac{{ - 3}}{8} = \frac{{ - 1}}{4} + \frac{{ - 3}}{8} = \frac{{ - 2}}{8} + \frac{{ - 3}}{8} = \frac{{ - 2 + \left( { - 3} \right)}}{8} = \frac{{ - 5}}{8}\)

d) \(\frac{2}{{12}} + \frac{7}{{24}} = \frac{1}{6} + \frac{7}{{24}} = \frac{4}{{24}} + \frac{7}{{24}} = \frac{{4 + 7}}{{24}} = \frac{{11}}{{24}}\)