For future reference, feel free to provide additional information here in the public forum. Waiting on just one person may not be particularly expedient.
See how important it is to give ALL the information?
I'm guessing these are prices.
These are indices.
Jan base month price index 100
Price index for June 107.9
The question wants a price.
What is the price of petrol in June
The idea of a "Price Index" is to pick a point and call that the base, then adjust everything else to conform to that base.
Jan Price * (some factor) = Jan Index = 100
We just need to discover the "some factor".
In this case, division should be sufficient: 100/88.9 = 1.12486
You should now be able to use this factor to convert between prices and indeces based on January.
Price to Index
Mar 89.9 (multiply by 1.12486) ==> 101.1
May 94.8 (multiply by 1.12486) ==> 106.6
Index to Price
June 107.9 (divide by 1.12486) ==> 95.9
Note: I'm still just guessing in at least three places. If we don't yet match the book, you'll have to provide better information.