Two numbers describe the stall, and they behave completely differently.
The first is the rent. It is ninety dollars a day, and it is ninety dollars whether she sells four hundred cups or none at all. It does not care. The second is the cost of the tea itself: two dollars a cup, every cup, and nothing when the stall is shut.
Against those she sets one number going the other way. Each cup sells for five dollars.
Draw the day on paper and you get two lines. One line is what she owes — it starts high, at ninety, and climbs by two with every cup. The other is what she takes — it starts at nothing and climbs by five. The second line is steeper, so however far behind it begins, it must eventually catch the first. The only question is where.
Three dollars of every cup goes towards the rent, so it takes thirty cups to cover it. Sell twenty-nine and she has lost money. Sell thirty-one and the day is hers. That crossing point has a name in her notebook and in every business textbook, and it is the number she watches all morning.
What matters for a test is the shape of the thought, not the tea. Two quantities, each changing at its own steady rate, and a question about the moment they are equal — that is one line crossing another, and it is the same picture whether the answer is a price, a distance or a day.