3. A store received candies at 13 lei/Kg in 50 boxes of $10 \mathrm{Kg}$ and $13 \mathrm{Kg}$. How many boxes are of $10 \mathrm{Kg}$, if after selling the candies, 7007 lei were collected?
Solution:

First, we find out how many kilograms of candies were bought:

7007 lei : 13 lei $/ \mathrm{Kg}=539 \mathrm{Kg}$. . . . $1 \mathrm{p}$

We use the method of false hypothesis:

Assume that there are only 10 kg boxes. . . 1 p

The total weight would be $50 \cdot 10 \mathrm{Kg}=500 \mathrm{Kg}$. . . $1 \mathrm{p}$

We have $539 \mathrm{Kg}-500 \mathrm{Kg}=39 \mathrm{Kg}$, the total difference . . . $1 p$

and $13 \mathrm{Kg}-10 \mathrm{Kg}=3 \mathrm{Kg}$, the difference for each 13 kg box. . . $1 \mathrm{p}$

We find the number of 13 kg boxes:

$39 \mathrm{Kg}$ : $3 \mathrm{Kg}=13$ boxes of $13 \mathrm{Kg}$. . . . $1 \mathrm{p}$

Finalization: we have $50-13=37$ boxes of $10 \mathrm{Kg}$. . . 1 p

Total: $7 p$