An aeroplane flies around a square the sides of which measure $100 \mathrm{kms}$. each. The aeroplane covers at a speed of $100 \mathrm{kms}$. per hour the first side, at $200 \mathrm{kms}$ per hour the second side, at $300 \mathrm{Kms}$. per hour the third side, and at $400 \mathrm{kms}$ per hour the fourth side. Use the correct mean to find the average speed round the square.