I teaspoon is equivalent to 5 ml and for 4 times 20ml is required in one day. If dose is for 7 days then 140 ml is required for whole 7 days. Now you say that amoxicillin granular are available as 125mg/ml. Then multiply total volume required that is 140 ml with 125. This is equal to 17.5 grams. Means you have to dispense 17.5 gram granules of drug to be dissolved in 140 ml for 7 days.