15 - 7 = 8 so there are many you can do like:
16 - 8 = 8
20 - 12 = 8
22 - 14 = 8
and so on, there are many.
We can write the following proportion between the milligram and milliliters:

Solving for x, we have

Intuitively, the original 1000mg/250ml solution tells us that the number of milliliters is one fourth of the number of milligrams.
So, if we want 700mg, we have one fourth of this value for the milliliters:

Answer:
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