It appears at first sight that there is too little information given but think again! This challenge provides good experience of problem solving.

Writing down the formulae for sums of series and averages is the first step, but there are two unknowns. Trial and improvement to find all the solutions would need unlimited time unless the range of values of n can be restricted. Consideration of the conditions of the problem, and a little work with inequalities, will cut down the possibilities for the values of n. After that it is a simple matter to find the solution using a spreadsheet or other computer program.