$\displaystyle { \left( 1 {3 \over 4} + 2 {4 \over 5} \right) *30 = \left( {4 +3 \over 4} + {10+4 \over 5} \right) *30 = \left( {7 \over 4} + {14\over 5} \right) *30 = }$ 

 $\displaystyle { \left( {7 \over 4} + {14\over 5} \right) *30 = \left( {7*5 + 14*4\over 4*5} \right) *30 = \left( {35+56\over20} \right) *30 = }$

 $\displaystyle { \left( {91\over20} \right) *30 = {91*30 \over20} = {91*3 \over2} = {273 \over2} = 136,5 }$