SQL & PL/SQL

Hello All,
I have to write a solution for this prob
Sample Data

Rate           PT              Months
2.2             300             60
2.4             350             12
2.5             400             12
2.5             400             12 
2.5            400              12
2.6            450              12
2.7            450              12

I need solution like this

Rate           PT              Months             Yrs
2.2             300             60                   1-5
2.4             350             12                   6
2.5             400             12                   7-10
2.6            450              12                   11
2.7            450              12                   12
NULL         NULL            NULL               NULL
NULL          NULL           NULL               NULL

Logic
If Col and Col2 does not changes over rows then there is merging of rows
I hop this will make sense.

Please help


Thanks a lot!