4. 15 bags of potatoes and 12 bags of flour weigh 1 ton 710 kg, with the weight of one bag of potatoes being 30 kg less than the weight of one bag of flour. How much does one bag of potatoes and one bag of flour weigh?
4. The first method of solving.
1) $30 \times 12=360$ kg - this is how much more 12 bags of flour weigh;
2) $1710-360=1350$ kg - this is the weight of potatoes and flour if they were of the same weight;
3) $15+12=27$ bags - the total number of bags if they were all the same;
4) $1350: 27=50$ kg - the weight of one bag of potatoes;
5) $50+30=80$ kg - the weight of one bag of flour.

The second method of solving. Let K be the weight of one bag of potatoes, M be the weight of one bag of flour. Then according to the condition:

$$
\mathrm{K}=\mathrm{M}-30,15 \mathrm{~K}+12 \mathrm{M}=1710 .
$$

We get

$$
\begin{gathered}
15 \times(M-30)+12 M=1710,15 M-450+12 M=1710 \\
27 M=1710+450,27 M=2160, M=80
\end{gathered}
$$

Answer: one bag of flour weighs 80 kg, and one bag of potatoes weighs 50 kg.