1. Three groups planted a part of a field. The first group planted $\frac{7}{20}$ of all the seedlings, the second group planted $\frac{3}{5}$ of the remaining, and the third group planted the remaining 260 seedlings. How many seedlings were planted in total?
Solution. Let's denote the total number of seedlings planted by x. According to the condition in the problem, we have $x-\frac{7}{20}x-\frac{3}{5}\left(x-\frac{7}{20}x\right)=260$, from which we find that $x=1000$.