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11.8

C = $\dfrac{1}{4}$.$\dfrac{7}{3}$.$12$ 

   = $\dfrac{7}{12}$.$12$ 

   = $7$ 

D = $\dfrac{3}{8}$.$56$.$\dfrac{25}{7}$.$(-4)$ 

   = $(\dfrac{3}{8}$.$-4)$.$(56$.$\dfrac{25}{7})$ 

   = $\dfrac{-3}{2}$.$200$

   = $-300$ 

11.9

A = $\dfrac{3}{7}$.$\dfrac{2}{5}$.$\dfrac{7}{3}$.$20$.$\dfrac{19}{72}$ 

   = $(\dfrac{3}{7}$.$\dfrac{7}{3})$.$(\dfrac{2}{5}$.$20)$.$\dfrac{19}{72}$ 

   = $1$.$8$.$\dfrac{19}{72}$

   = $\dfrac{19}{9}$

B = $\dfrac{6}{7}$.$\dfrac{8}{13}$$+$$\dfrac{6}{13}$.$\dfrac{9}{7}$$-$$\dfrac{3}{13}$.$\dfrac{6}{7}$

   = $\dfrac{6}{7}$. $(\dfrac{8}{13}$$-$$\dfrac{3}{13})$$+$$\dfrac{6}{13}$.$\dfrac{9}{7}$ 

   = $\dfrac{6}{7}$.$\dfrac{5}{13}$+$\dfrac{6}{13}$.$\dfrac{9}{7}$

   = $\dfrac{30}{91}$$+$$\dfrac{54}{91}$

   = $\dfrac{84}{91}$ 

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