The weight of four plums and 1 apple is equal to 1 peach. Replace a peach with the following relationship.

13 plums = 2 apples and 1 peach,

So the weight of 13 plums = the weight of 2 apples and 4 plums and 1 apple,

That is 13 plums =3 apples and 4 plums.

So 3 apples =9 plums.

So 1 apple =3 plums.

So the weight of 4 plums and 1 apple =7 plums.

And the weight of 4 plums and 1 apple = 1 peach.

So seven plums are as heavy as a peach.