Rent is due whether you sell one glass or a hundred. So every day starts in a hole, and the only question that matters before you open is: how many do I have to sell just to climb out? That number is break-even, and it is a linear equation doing an honest day's work.
Fixed costs do not change with how much you sell — the rent for your spot. Variable costs change with every unit — the lemon, sugar and cup in each glass.
Break-even is where what you keep from each sale has finally paid off the fixed cost.
Break-even quantity = fixed cost ÷ profit per unit.
If rent is ₹20 and you keep ₹5 on each glass, you need 20 ÷ 5 = 4 glasses before the day is worth anything. Glass five is the first rupee you actually earn.
Written as an equation: 5n − 20 = 0, so n = 4. This is precisely the Class 8 linear equation chapter, with a reason to exist.
Now raise the rent to ₹60 for a busier street. Break-even jumps to 12 glasses. The better location is only the better decision if it brings at least 8 more customers — which is a calculation you can do before you move, not after.
In Lemonade Empire your child runs a lemonade cart and uses this exact concept to decide what to buy and what to charge. Free, no signup, plays in a browser.
▶ Play Lemonade Empire free