Faction food

Hi!

First of all forgive me my crippled english - its not my native language.

But to the point. I am wondering what does "add x per grain to faction food" realy mean? If i have 3 citys with 2, 3 and 4 grains and i'll build a grocer in the 3 grain city? What then:

a ) 1'st town gets +10 food (2 grains x 5 food from grocer), 2'nd gets +15 (3 grains x 5 food from grocer) and 3'rd one gets +20 (4 grains x 5 food from grocer)

or maby

b ) every town gets +15 food (5 food from grocer x 3 grains from city a gocer was build in)?

Can you help me? And before you will write things like: "Use search you morron!" i asure you: I uset it. Didn't find anything.