Index number

kindly solve;

In 1996 the avg price of a commodity was 20% more than in 1995 but 20% less than in 1994; and more over it was 50% more than in 1997 t price realtive using 1995 as base (1995 price relative 100) Reduce the data is:

Ans is : 150,100,120,80 for (1994-97)

kidly guide how the index number for theyear 1997 is arrived at 80?