Answer :

ratio and proportion

6/33= 5/x
cross multiply
6x= 165
 x= 27 1/2 m^2 with 5 bags

Hope this helps
6 bags of plaster do 33m squared
1 bag of plaster do      33/6 m  squared
5 bags of plaster do     33*5/6 m squared
                                = 27.5 m squared

So your answer is 27.5 m squared