I came across this problem some years ago in a book (which I won't mention, and request others to not mention if they know). The solution is quite elegant and difficult to arrive at by mere trial and error. And I couldn't think of a proper way to solve it. Still, give it a try.
Once, there was an egg seller. She had $90$ eggs which she wanted to sell in the market. So, she sent her $3$ girls to the market. She gave the eldest and the cleverest girl $10$ eggs, the second girl $30$ eggs and the youngest one $50$. She told them, "You better decide for the price of the eggs among yourselves. But the three of you should stick to one price. You may change the price more than once, but the three of you must change it together. Also, I expect that the three of you earn the same amount of money each by selling your respective eggs. And you must bring home not less than $90$ units of money."
The egg seller told them they should not bring home less than $90$ units. The eldest girl thought of an ingenious way and explained her sisters her idea. In the evening, when they came home, each girl had earned $30$ units by selling her respective lot of eggs, and they together had brought home exactly $90$ units.
Can you think of what idea the eldest girl used?
Edit: Let me clarify the situation a bit more. The three girls occupy different places in the market. Since the eldest girl told them the plan, they have no connection whatsoever till evening. So, they cannot go and meet or sell their eggs to each other. It is not necessary to have a price per egg. Eg, $5$ units per $3$ eggs is a totally acceptable price. With their respective transactions, each of them earns $30$ units. All the eggs are sold in the end.
Edit 2: Also, if the girls decide to sell in bundles, they must sell until their bundles are finished. Eg, if they set the price to $5$ per $9$ eggs, the eldest will sell her $9$ eggs, the middle one will sell her $27$ eggs, the youngest will sell her $45$ eggs before changing the price again. This is one thing I should have told earlier. Sorry. O:-)
If any more doubt, please add in the comments.