## 278. Math Puzzle $7 / 88$

Rolli bought bottles of lemonade for a Pioneer party at the store for 20.79 marks. After emptying them, he returns them and receives 30 pfennigs for each one. With the money he receives, he buys ten new bottles of lemonade and still has 40 pfennigs left.

How many bottles did Rolli buy on the first purchase?
We denote with $x$ - the number of bottles in the first purchase, $y$ - the price for one bottle (excluding deposit) in pennies

$$
x(y+30)=2079 \quad ; \quad 10(y+30)=30 x-40
$$

We obtain the following quadratic equation: $3 x^{2}-4 x-2079=0$ with the solutions $x_{1}=27$ and $x_{2}=-\frac{77}{3}$.

Rolli bought 27 bottles in the first purchase, and one bottle costs 47 pennies without the deposit.