How to calculate Index?

Say you have something like this:
Year................1972.......1980......1986
Price Index........337........705
(Base year 1949)

Price Index...................................186
(Base year 1980)

So how do you calculate the price index for 1986 with base year 1972?
I tried (705/337) x 186 and got 389.12, but the answer in the book is 388.3?
(I don't think the answer in the book is wrong, because there're a couple of similar questions where I got the wrong answer by an equally small margin after calculating like I did in here.)
Any help?