1. 7 plums. Put on one side of the scale 3 peaches and a pear and on the other side - 10 plums. Then, by second condition, the scale will be in equilibrium. Using the first condition now, let's substitute the pear with a peach and 6 plums. Therefore, on one side of the scale we'll have 4 peaches, 6 plums and on the other - 10 plums. So the peach and plum have the same weight. Then, by the first condition, instead of 1 peach and 6 plums we can take 7 plums, which will be in equilibrium with one pear.