The count of landline telephone connections in a town falls by 20% every year. If the town has 51,200 connections today, how many did it have three years ago?

Each year the count is multiplied by 0.8, so a count $C$ three years ago satisfies $Cx0.8\times 0.8\times$ 0.8 = 51,200, that is $Cx$ 0.512 = 51,200 and $C=1,00,000$. Check: 100000 -> 80000 -> 64000 -> 51200. 1,28,000 treats three 20% falls as one 60% fall (51200/0.4) - repeated falls compound, so 51.2% survives, not 40%. 64,000 and 80,000 step back only one year and two years.