Question: An IV bag contains $1.5$ litres of saline. If $650$ml has already infused, how many millilitres ($ml$) remain?
1. **State the problem:** We have an IV bag with $1.5$ litres of saline. $650$ millilitres have already infused. We need to find how many millilitres remain.
2. **Convert litres to millilitres:** Since $1$ litre = $1000$ millilitres, convert $1.5$ litres to millilitres:
$$1.5 \times 1000 = 1500 \text{ ml}$$
3. **Calculate remaining volume:** Subtract the infused volume from the total volume:
$$1500 - 650 = 850 \text{ ml}$$
4. **Answer:** There are $850$ millilitres remaining in the IV bag.