Question: A liquid medication must be diluted in a ratio of $1$ part medicine to $4$ parts water. If the total volume of the mixture is $200$ml, how much medicine is in it?
1. **State the problem:** We need to find the amount of medicine in a mixture where the ratio of medicine to water is $1:4$ and the total volume is $200$ml.
2. **Understand the ratio:** The ratio $1:4$ means for every $1$ part of medicine, there are $4$ parts of water.
3. **Calculate total parts:** Total parts = $1 + 4 = 5$ parts.
4. **Find the volume of one part:** Volume of one part = $\frac{200}{5} = 40$ ml.
5. **Calculate medicine volume:** Medicine volume = $1$ part $\times 40$ ml = $40$ ml.
**Final answer:** The mixture contains $40$ ml of medicine.