Economics p1 help plz

[Samii]

The retail price index of a country rose during a year from 200 to 240.
What was the annual rate of inflation ?
Plz tell me how to calculate RETAIL PRICE INDEX

 

[Shahnam]

RPI = [(change in index)/(base year price)]x 100
if 200 is of base year
then; (40/200) x 100
Rate of inflation = 20%