If the result of measuring blood pressure of 300/240 is necessary to ensure good blood pressure monitor, the easiest way to carry out a control measurement of BP on the opposite arm.
Likely that was incorrectly located the blood pressure cuff (not over the artery on which the measurement is made) or the hand and heart of the patient are not at the same level (as recommended by the measurement methodology).
If the result of the blood pressure measurements are confirmed by human BP 300 over 240 – this may cause weakness, headache and loss of consciousness. With this blood pressure you better consult a doctor for further advice, because remotely to determine the cause of the blood pressure of 300/240 is very difficult.
Important: This blood pressure is NOT NORMAL and can cause a hypertensive crisis. You need to go to the hospital for medical help or take medication to lower blood pressure.