Great example of how the GMAT can test something you already know in a slightly tricky Integrated Reasoning way. The important thing is to not waste too much time on a question that's so straight forward and covers a concept you should know well.
If the price of chair P is 60$ and the price of chair Q is 90$, the average must be somewhere in the middle. If the average is 75$, then there is an equal amount of each. If the average is 70$ as stated here, then you need two chairs of P brand (down by 10$ each) for every one of Q (up by 20$ each). This means you need twice as many P's as Q's, so you just look for the numbers that are in a 2-1 ratio.
Always start at the top, as you can see that doubling each number will get you out of the range of possible answers fairly quickly. Multiplying by two is also easier than dividing by two for 99.44% of people (stat may be a little arbitrary) so stick to multiplying rather than dividing. 28 doubles to exactly 56, so you need 56 of the cheaper chair and 28 of the more expensive chair. There's no need to do trial-and-error in a question like this, understanding the concept and applying it should take you about a minute or so to get the right answer.
Hope this helps!
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