in average? on average? [closed]
If you do a Google ngram search you'll realize starting in the '60s more people had the tendency to use the preposition on before average although a search in Corpus Contemporary American English would suggest that the expression in average can still be seen in published books nowadays, but they are normally used with average as an adjective in sentences like "There is an increase in average sale."
Therefore I would advice you to go for the expression "on average" instead.
"on (or : on an) average" is sole in use.