If the force between two point charges of charge 'Q1' and 'Q2' which are a distance 'r' apart is 'F' then what would the force be if the charge of 'Q1' is tripled and the distance between them doubled?

We know from Coulombs law that the force between two point charges is directly proportional to the product of the magnitudes of each charge and inversley proportional to the square of the distance between them. So tripling one of the charges will multiply the force by 3 and doubling the distance will divide the force by 4 as the distance is squared. So the answer is 3/4F.