Answer :
1st week= 20 walls
2nd week= 20+20=40 walls
3rd week= 20+20+20=60 walls
4th week= 20+20+20+20=80 walls
Therefore, in 4 weeks 80 walls in total will be painted
1st week= 20 walls
2nd week= 20+20=40 walls
3rd week= 20+20+20=60 walls
4th week= 20+20+20+20=80 walls
Therefore, in 4 weeks 80 walls in total will be painted