$\text{a)}$

$19+13+11=\left(19+11\right)+13=30+13=43$

$\left(26+34\right)+17=60+17=77$

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$\text{b)}$                          

$15+16+45=\left(15+45\right)+16=60+16=76$

$\left(53+27\right)+18=80+18=98$

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$\text{c)}$  

$16\cdot 5\cdot 0=0$

$4\cdot 27\cdot 5=\left(4\cdot 5\right)\cdot 27=20\cdot 27=540$

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$\text{d)}$  

$25\cdot 18\cdot 40=\left(25\cdot 40\right)\cdot 18=1000\cdot 18=18000$

$50\cdot 36\cdot 20=\left(50\cdot 20\right)\cdot 36=1000\cdot 36=36000$