Hello!
If you do not know 1 question;
100-20=80
If you do not know 2 question;
20+20=40 100-40=60
If you do not know 3 question;
20+20+20=60 100-60=40
If you do not know 4 question;
20+20+20+20=80 100-80=20
If you do not know 5 question;
20+20+20+20+20=100 100-100=0