a) $\frac{1}{3}$ . $\frac{5}{7}$ - $\frac{7}{24}$ . 36
= $\frac{5}{21}$ - $\frac{21}{2}$
= $\frac{10}{42}$ - $\frac{441}{42}$
= $\frac{-431}{42}$.
b) 70,5 - 528 : 50/2
= 70,5 - 528 : 25
= 70,5 - 21,12
= 49,38
c) 5/9 . 7/13 + 5/19 . 9/13 + 5/9 .3/13
= 5/9(7/13 + 3/13) + 5/19 . 9/13
= 5/9 . 10/13 + 5/19 . 9/13
= 13/18 + 45/247
= 3283/4446