If D is the number of days late then, for D≥1 the charge C=10+5(D-1)=5+5D=5(D+1); for D<1, C=0.
For D=21, C=5*22=Rs110.
If Rs5 is to be charged in addition to Rs10 on days after the first day, the formula is different: C=10+15(D-1) for D≥1. So C=10+15D-15=15D-5; C=5(3D-1). When D=21, C=5*62=Rs310. If D<1, C=Rs0.