$a)18\cdot 2\frac{6}{7}+12\cdot 4\frac{2}{9}\approx 18\cdot 3+12\cdot 4=54+48=102$

$b)876,42+564,76-827,123\approx 876+565-827=1441-827=614$

$c)\left(16,64+76,32\right)\cdot \left(81,81-27,87\right)\approx \left(17+76\right)\cdot \left(82-28\right)=93\cdot 54\approx 90\cdot 50=4500$