Volume of milk bought by each regular customer = 500 ml
Volume of milk for 65 customers = 65 × 500 ml = 32500 ml
Volume of milk as backup = 5 litres = 5000 ml
Total volume of milk to be stocked for the day = 32500 + 5000
= 37500 ml
= 37 litres 500 ml
So, Sonu has to stock up 37 litres and 500 ml of milk for the day.