Patient weighs 15 kg.
10% dehydration = 15000 x 0.1 = 1500 ml
Already given 300 ml leaving 1200 ml deficit
Maintenance fluids = 100 ml/kg for first 10 kg body wt. then 50ml/kg for next 10 kg = 1250 ml over 24 hrs = 625 ml over 12 hours.
Total over next 12 hours = 1200 + 625 = 1825 ml = 152 ml/hr
BUT do not forget any other losses such as diarrhoea and vomiting.
Original serum sodium was normal at 139 mmol/l.
Therefore maintenance fluids should be given as 0.45% saline + 5% dextrose and replacement of further losses should be given as 0.9% saline.