You are on the paediatrics ward and a three year old child is prescribed ibuprofen orally 5mg/kg three times a day when necessary. You weigh the child and they are 18kg. On the drug round the child has a temperature and it is appropriate for you to give a dose of Ibuprofen. If you have a 100mg/5mL suspension, what volume of the suspension do you have to draw up into the oral syringe for administration?