Fred had just started work in Hotel Plush and was dealing with a demanding but important guest. “Pour my coffee, boy! I want exactly 200ml coffee to which you will add enough full cream milk to make it 20% milk. Do it correctly and you may go about your other duties; if you get it wrong, I will call the manager,” screeched the darling lady.
It was all Fred could do to stop his hands from trembling as he started to pour the coffee. He poured 200ml into the measuring jug, (he keeps one about his person at all times for such eventualities), then started to think about the milk. 20% of 200ml is 40ml but if he poured 40ml in, the volume would be 240ml, so it wouldn’t be 20% milk any more. How much milk should he add?
Answer
The only volume we know is for the coffee, 200ml, so that’s the starting point. We also know the final coffee is to be 20% milk, but that also means it is 80% coffee , so 80% = 200ml. That also means 20% = 50ml (80%/4=200ml/4), so he needs to add 50ml milk. Whether he puts anything else in there as well is between Fred and his conscience.
