average monthly costs for the past three months and forcast by 1%

I need a sum (function) to take the average of the three preceeding cells and increase it by 1%. The idea is to take my average monthly advertising costs for the past three months and forcast it for the next month including a 1% increase. I can do the average with this (=AVERAGE(O4:Q4)) but I do not know how to add the 1%. Help!!!

Thanks. I will remember next time