2. Grandpa Kosta needs to plow and sow his field with seed material-wheat. He plowed the field in 3 days. On the first day, he plowed 20% of the entire field and another 16 hectares, on the second day 30% of the remainder from the first day and another 20 hectares, and on the third day he plowed 75% of the remainder from the previous day and the last 30 hectares. How many tons of seed material-wheat does Grandpa Kosta need to buy if 3.5 kg of wheat is needed for sowing one hectare?
Solution. First method. The last 30 decares are 25% of the area plowed on the third day, which means that on the third day, Grandpa Kosta plowed $4 \cdot 30=120$ decares, which remained unplowed after the second day. Since on the second day, Grandpa Kosta plowed $30\%$ of the remainder from the first day and 20 more decares, we get that after plowing $30\%$ of the remainder from the first day, 120 + 20 = 140 decares remained unplowed, which means that $70\%$ of the remainder from the first day is 140 decares, i.e., the remainder after the first day is $\frac{140}{70} \cdot 100=200$ decares. The latter means that after plowing $20\%$ of the entire field, 200 + 16 = 216 decares remained, which is $80\%$ of the entire field. Thus, the entire field had $\frac{216}{80} \cdot 100=270$ decares. Finally, since 3.5 kg of wheat is needed to sow 1 decare, we get that to sow the field, Grandpa Kosta needed to buy $270 \cdot 3.5=945$ kg of wheat.

Second method. Let the field have $x$ decares. On the first day, $0.2x + 16$ decares were plowed, so $x - (0.2x + 16) = 0.8x - 16$ decares remained unplowed. On the second day, $0.3(0.8x - 16) + 20 = 0.24x + 15.2$ decares were plowed, so $0.8x - 16 - (0.24x + 15.2) = 0.56x - 31.2$ decares remained unplowed. On the third day, $0.75(0.56x - 31.2) + 30 = 0.42x + 6.6$ decares were plowed, which completed the plowing of the entire field, so $0.42x + 6.6 = 0.56x - 31.2$. The last equation is equivalent to the equation $0.14x = 37.8$, from which we get $x = 270$ decares. Finally, since 3.5 kg of wheat is needed to sow 1 decare, we get that to sow the field, Grandpa Kosta needed to buy $270 \cdot 3.5 = 945$ kg of wheat.