EXAMPLE 2
lets assume that the dose will be always given like20microgram/kg/hour 
the weight of the child will also be given-3--4 kgs
the concentration of drug will also be given
like one ampule(1ml) has .5 ,1 ,2 0r 5mg of the drug.
the usual question is to calculate in how much normal
saline we should dilute this to give it for 24hours??

total dose for the given child/patient in one hour----dose/kg/hr multiply by given weight
multiply this new figure with 24====total dose for that person in 24 hour
now calculate how many ampoules (mls)will contain this quantity of drug which is needed for 24 hours -ASSUME = Y
Therefore the total quantity of drug needed for 24 hrs is Y micrograms
hours for that particular patient is Y micrograms
and if the the concentration of the drug is available as O.5mg(500microgram)per ml
so 500 microgram is contained in fluid=1ml
1---------------------------------1/500
Y micrograms-----------------------------1/500ml X Y micrograms= A ml

so the drug need for 24 hour for that person is in
A ml of the given concentration.

now we need to give the medicine at 1ml/hr rate, it
means we need 24 ml of total solution

of this A ml is essential coz it has the drug
the remaining ( 24-A )ml will be normal saline
so we need 24 - A ml of normal saline

it will make a solution which will contain
Y microgram of the drug.
and if give it 1 ml per hour we will be giving
the required concentration / hr