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2ml

Step-by-step explanation:

50mg of some potent agent has to be given every 12 hours.

there is a solution that has a concentration of that agent of 125mg/5ml

we need to administer some part of this solution, which we cannot (or should not) change in its structure.

that means the ratio of agent to overall solution stays the same, no matter how much of the solution we administer.

all we need to do is to transit the ratio of 125/5 to represent 50/x (maintaining the said ratio).

in other words, we need to find how many ml we need to administer, so that 50mg of the agent enter the body.

so,

125/5 = 50/x

125x/5 = 50

25x = 50

x = 50/25 = 2

2ml of the solution needs to be administered every 12 hours.