Question 1938
to be honest, no formula needed, this is just a ratio question.

Start with what you know, then find what 1 is, then find what you are asked for.

so, we know from 200 there would be 3
so, we infer therefore that a sample of 1 would have (3/200) buyers (yes, this is nonsensical, but it gets you there.
so we imply again that in a sample of 280 million (280,000,000) there would be (3/200)*280000000 = 4,200,000 buyers.

There may well be an "instant" method, but this 3 line approach can be used for unit conversion questions - they are tackled all in the same way. It has never let me down and i never confuse myself and there is no "formula" to remember either :-)

jon.