The answer to this question would be 10ml.
Assuming the potassium chloride concentration is 20mEq/5ml, then the calculation would be: 40 mEq / (20mEq/5ml)= 10ml.
Be careful when administering KCl through intravenous line since KCl can irritate veins. Using a bigger central vein is recommended. If you need to administer big amount fast.