please explain through excel step by step

An ABS has been structured with 80% in the senior tranche and 20% in the subordinated tranche. The shifting interest schedule is as follows: (a) Assume that in year 1 the pro rata shares are 80% and 20%, what are the prepayment percentages for each tranche? (b) Assume that in year 4 the pro rata shares are 75% and 25%, what are the prepayment percentages for each tranche? (c) Assume that in year 6 the pro rata shares are 70% and 30%, what are the prepayment percentages for each tranche? (d) Assume that in year 9 the pro rata shares are 65% and 35%, what are the prepayment percentages for each tranche