Dilution calculation question

Hello, I have a type of question that I find difficult to answer, here is an example of what I need to do:

"What volume of phenytoin suspension 30mg/5ml is required to be added to the diluent to obtain 150ml of phenytoin suspension 20mg/5ml?"

If someone could give me an idea of the method required to go about answering this kind of question, it would be appreciated!

Thanks in advance.

What volume of phenytoin suspension 30mg/5ml is required to be added to the diluent to obtain 150ml of phenytoin suspension 20mg/5ml?"

1st determine the mass of phenytoin in 1 ml of each phenytoin suspension.

30mg/5ml = 6 mg/ml
20mg/5ml = 4 mg/ml

2nd determine the mass of phenytoin in 150 ml of the 20mg/5ml phenytoin suspension.

150 ml * 4 mg/ml = 600 mg


3rd determine the volume of the 30mg/5ml phenytoin suspension that contains 600 mg of phenytoin.

600 mg ÷ 6 mg/ml = 100 ml

100 ml of 30mg/5ml phenytoin plus 50 water = 150 ml of 20mg/5ml phenytoin suspension.

you have 30/5 mil you need 20/5 ml so you need to dilute by 2/3 if you have final volune 150 mls then 1/3 water 2/3 sol

50mls water + 100 mils sol