For 12 mm thick cement plastering 1 : 6 on 100 sq.m new brick work, the quantity of cement required, is

Net volume of mortar=100*12/1000=1.2cub.m
Add 30%mortar for joint filling =30/100*1.2+1.2=1.56 this is the total wet vloume
Now add 30%to wet vloume to obtain dry volume of mortar hence
Dry vloume =30/100*1.56+1.56=2.028
And now volume of cement =2.028/1+6*1=0.274

Solve