This is easy: little less than 16,7 liters
A final 30% v/v solution could be made by adding 30 L of ethanol to 70 L of water.
Being 30 L six times more than 5 L on hand, we must add to it a sixth of 70 L of water, that is 70/6 = 11,7 L, giving a total volume of 5 + 11,7 = 16,7 L.
Unfortunately the real volume will be some 2% less because of a contraction of the solution.
Your teacher should give you tables of experimental density data of alcohol solution at several concentrations, to determine the precise amount of final solution, in the case he or she would pretend that.
Hope that was useful.